The AndyMark chassis is in an assembly. The assembly is simply a reference of several parts. Whenever you open the assembly, it must be able to find the parts, otherwise it asks you to "Resolve Link." For something like the AM chassis, you can just click skip for all of the parts that it is missing. Once you have skipped through all of the parts, you will have a workable CAD of the base.
The parts that Inventor is looking for are all bolts.
Alternatively, you could just download the CAD from AM.
http://andymark.biz/am-0453.html and not have any problems.