Stephan,
I would recommend using VB.NET instead of VBA. Inventor 2011 includes samples of VB.NET projects that build forms for entering dimensions, along with the iLogic rules to connect to them. These are installed in the directory of the "iLogic 2011 Samples" Inventor project.
But it is also possible to hook up to a VBA form. You can use the Inventor API (the Parameters collection on the PartComponentDefinition or AssemblyComponentDefinition object) to access all parameters. You might not need to use the iLogic API from the form.
You can see some documentation for the interfaces in Autodesk.iLogic.Interfaces.dll if you add a reference to this dll and then use the Object Browser in Visual Studio. The iLogic API is accessed on the Autodesk.iLogic.Interfaces.IiLogicAutomation interface.
It's not easy to bring up a custom form automatically during the Place iLogic Component command. One way to do it would be to have a True/False parameter named ShowForm. If you change it to True it would fire a rule that would show the form.
Mike Deck
Autodesk
Mike Deck
Software Developer
Autodesk, Inc.