There is no way to set an option that will toggle an edit field into a required text field, however with a custom action and some InstallScript we can prevent the next dialog from being advanced and display a message asking for the field to be populated.

1) What you'll want to do is create a InstallScript function that calls MsiGetProperty() on the property(ex TEXT) that the edit field populates. You can create an InstallScript function under Behavior and Logic->InstallScript. Then in the middle pane right click Files->New Script File. A function MyFunction() should be created in the setup.rul. In the function you'll want to check if the edit field's property(value returned by MsiGetProperty) is empty or populated and then do MsiSetProperty() on a second property(ex ADVANCENEXT) that will determine whether or not the dialog should advance.

2) Next associate the created installscript function with a custom action. This can be done in th Custom Actions and Sequences view, right click Custom Actions in the middle pane and New InstallScript. Set the Function Name to the name of the function created in Step 1).

3) Setup the message to be displayed when field is empty. Create another InstallScript function that calls the InstallScript funtion MessageBox() and associate that with another custom action.

4) Navigate to the dialog with the edit field under the Dialogs view, go to its behavior. Add a condition to the NewDialog event on the Next PushButton that validates against the second Property(ex ADVANCENEXT). Add a DoAction event before the NewDialog that calls the custom action created in Step 2. Also add a DoAction to call the MessageBox custom action of Step 3 with a condition that validates against the second Property (ex ADVANCENEXT).

In essence, when the Next button is pushed it will call a custom action that checks to see if the Text field is empty and sets a property that allows/prevents advancement.

Here is example code for the InstallScript Custom action.

MsiGetProperty(hMSI,"TEXT",szValue,szSize);
if (szValue = "") then
SprintfBox(INFORMATION,"Enter Text","Please Enter a Value for the Text Field");
MsiSetProperty(hMSI,"ADVANCENEXT","0");
else
SprintfBox(INFORMATION,"Value Set","Value Set");
MsiSetProperty(hMSI,"ADVANCENEXT","1");
endif;