May I suggest reading some of the examples from the Qt documentation ?

Typically updating your line edit content from another widget should be done using signals and slots (Unless I misunderstood your code). Or if SelectFile is a dialog to select a file (like the name suggests) why not use QFileDialog from BasicGui and directly update the line edit ?

@public:
explicit SelectFile(QWidget *parent = 0);
~SelectFile();
BasicGui *basicgui;
@

Then I made a call to basicgui->displayProgram()

displayProgram() is from another class (Ui::BasicGui) so when I do that it says that displayProgram is not a member of the class that I am making the call in (which in this case is called Ui::SelectFile).

That's not allowed[/quote]

OK, I made the function non-static, but am now getting a function saying "error: C2352: 'BasicGui::displayProgram' : illegal call of non-static member function" when trying to call the function from another class/file with the following code:

@ BasicGui::displayProgram(aPath);@

That's not allowed

@#ifndef BASICGUI_H
#define BASICGUI_H

#include <QMainWindow>
#include <QtGui>
#include <QtCore>

namespace Ui {
class BasicGui;
}

class BasicGui : public QMainWindow
{
Q_OBJECT

public:
explicit BasicGui(QWidget *parent = 0);
~BasicGui();
static void displayProgram(QString);

private slots:
void on_selectProgramButton_clicked();
void on_loadProgramButton_clicked();

private:
Ui::BasicGui *ui;

};

#endif // BASICGUI_H@

@ ui->programTextEdit->setPlainText(line);
ui->runButton->setEnabled(true);
ui->pauseButton->setEnabled(true);@

in the constructor, I get no build error. The errors only come up if I throw it in a separate function within my source file. Why is that?

Did you do this already? If not, try it please.

@class Ui_BasicGui
{
public:
QWidget *centralWidget;
QGroupBox *groupBox;
QGridLayout *gridLayout;
QHBoxLayout *horizontalLayout;
QRadioButton *radioButton;
QRadioButton *radioButton_2;
QRadioButton *radioButton_3;
QPushButton *machineOnButton;
QPushButton *machineOffButton;
QGroupBox *groupBox_4;
QGridLayout *gridLayout_5;
QGroupBox *groupBox_2;
QFormLayout *formLayout;
QLineEdit *lineEdit;
QGridLayout *gridLayout_2;
QPushButton *zPlusButton;
QPushButton *yPlusButton;
QPushButton *xMinusButton;
QPushButton *xPlusButton;
QPushButton *yMinusButton;
QPushButton *zMinusButton;
QGroupBox *groupBox_8;
QGridLayout *gridLayout_3;
QRadioButton *radioButton_4;
QRadioButton *radioButton_5;
QGroupBox *groupBox_6;
QGridLayout *gridLayout_7;
QTextEdit *programTextEdit;
QPushButton *selectProgramButton;
QGroupBox *groupBox_7;
QGridLayout *gridLayout_8;
QSlider *horizontalSlider;
QPushButton *eStopButton;
QWidget *layoutWidget;
QVBoxLayout *verticalLayout_4;
QGroupBox *groupBox_3;
QGridLayout *gridLayout_10;
QGridLayout *gridLayout_4;
QLabel *label_4;
QLabel *xPosActualLabel;
QLabel *label_5;
QLabel *yPosActualLabel;
QLabel *label_6;
QLabel *zPosActualLabel;
QGroupBox *groupBox_5;
QGridLayout *gridLayout_11;
QGridLayout *gridLayout_6;
QLabel *label_7;
QLabel *xVelActualLabel;
QLabel *label_8;
QLabel *yVelActualLabel;
QLabel *label_9;
QLabel *zVelActualLabel;
QWidget *layoutWidget1;
QHBoxLayout *horizontalLayout_3;
QPushButton *runButton;
QPushButton *pauseButton;
QPushButton *loadProgramButton;
QToolBar *mainToolBar;
QStatusBar *statusBar;@

I did include ui_basicgui.h in my project. It wasn't <QTextEdit> either. Here is the code again:

@#include "basicgui.h"
#include "ui_basicgui.h"
#include "loadfile.h"
#include "ui_loadfile.h"
#include "ui_selectfile.h"
#include "selectfile.h"
#include <QtDebug>

BasicGui::BasicGui(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::BasicGui)
{
ui->setupUi(this);
}

BasicGui::~BasicGui()
{
delete ui;
}

void BasicGui::on_loadProgramButton_clicked()
{
LoadFile loadFileDialog;
loadFileDialog.setModal(true);
loadFileDialog.exec();
}

void BasicGui::on_selectProgramButton_clicked()
{
SelectFile selectFileDialog;
selectFileDialog.setModal(true);
selectFileDialog.exec();
}

void BasicGui::displayProgram(QString aPath)
{
QFile programFile(aPath);
programFile.open(QIODevice::ReadOnly);
QTextStream textStream(&programFile);
QString line = textStream.readAll();
programFile.close();

qDebug() << line;
ui->programTextEdit->setPlainText(line);
ui->runButton->setEnabled(true);
ui->pauseButton->setEnabled(true);

}
@

Please encode your code in coding tags that will make it readable.

Your are missing
@#include <QTextEdit>@

in your cpp file so the compiler doesn't know what it is

edit: getting closer :D
edit2: I missread the error, you are missing your ui_xxx include. The compiler doesn't know what ui is.

the error message tells you that the compiler doesn't know the type of "programTextEdit". Did you include your ui-header file?