I received the above error whenever I tried to load my code in via the catalog browser in the administrative section (the individual features worked when I loaded them in via the LoadSpec utility). The above error is pretty self explanatory though (I was having one of those I’ve started at the screen for too long days). Basically, the site isn’t finding the assembly. In my case, the assembly was there which puzzled me until I realized that the name I used in my XML spec and the name of my actual assembly were different (Training.Core.Catalog.dll which was in my XML vs. Training.Catalog.dll which was what was actually in my bin directory). The morale of the story is to check the DLL references in the XML specs and make sure the file name is the actual filename of the assembly you’re targeting (and that it exists in the bin directory of your web-site.. in my case in bincustom).
Also, make sure that this line is uncommented (or exists) in your web.config:
<probing privatepath="bincustom" />