I tried to use Process Editor (MSVS 2010, Power tools 12/2011) to open the AcceptanceTest.xml WIT definition which I extracted from the processtemplate.zip for SfTS v3.0.4281.0.
I get an error that says: This file does not contain a Work Item Type definition.
If I extract the entire contents of the processtemplate.zip and then use Process Editor to open the process template (processtemplate.xml) then open the Acceptance Test work item it opens just fine.
I found that if I exported the Acceptance Test work item from a project (Tools > Process Editor > Work Item Types > Export WIT) I could open that file without a problem.
When I compared the two versions of the WIT definition I found that the file in the process template defines the WORKITEMTYPE item as follows:
<WORKITEMTYPE xmlns="" name="Acceptance Test">
whereas the exported template had the following form:
<WORKITEMTYPE name="Acceptance Test">
Note that the 'xmlns=""' attribute is not in the exported file.
I edited the AcceptanceTest.xml in the unpacked processtemplates.zip and removed the xmlns="" attribute. Once I did that I could use "Open WIT from file".