To use a Dynamics GP modified form in a .net VSTMenus project, you must run a DAG.exe utility program from a command line. This DAG.exe was installed when you installed the VSTMenus software code.
1) Go to a command prompt
2) Run this:
cd\Program Files (x86)\Microsoft Dynamics\GP 2013 vs Tools SDK\
dag.exe 0 "c:\program files (x86)\Microsoft Dynamics\Gp2013\Dynamics.set" /F /N:Dynamics
3) This creates two files:
Application.Dynamics.ModifiedForms.dll
Application.Dynamics.ModifiedForms (an XML document)
4) Copy these 2 files from the "GP 2013 vs Tools SDK" folder into your main GP application folder (typically c:\program files (x86)\Microsoft Dynamics\Gp2013).
5) These 2 files will also need to be copied to every PC running Dynamics GP.
See http://msdn.microsoft.com/en-us/library/cc543572.aspx for more on the DAG.exe utility.