Visual Build Professional currently doesn't provide an entry field specifically for entering conditional compilation values, however you can easily specify them in the "additional values ..." field on the Options tab using the /d flag.
In your case you would simply enter
/d isDebug=0 in the additional values field on the Options tab.
Here is the MS website documenting this feature and related command-line vb compiler flag:
http://msdn.microsoft.com/library/de...ompilation.asp
Note: this won't actually alter the Visual Basic project, but will simply override the value specified in the project file (which remains unchanged). I assume this is what you are requesting.