Your .dna file works perfectly on my Excel 2010 - both using the Excel-DNA v0.29 and the latest source check-in.
The crashing under Excel 2007 sounds very unusual. What version of Excel-DNA are you using?
* Check under File->Options->AddIns, Manage: Disabled Items. Sometimes, while developing, your add-in crashes a few times, and Excel sends it to the dog box. This affects subsequent loading
* Do any of the sample add-ins with Excel-DNA work in your Excel?
* Is the =AddThem(2,4) function that you also have in there registered successfully?
* Are you perhaps using Excel-DNA v0.29, and running Excel 'As Administrator' in a UAC elevated context? This had a bug that was fixed subsequent to the v0.29 release, which would affect the ribbon loading.
These are some initial ideas.
Let me know if this helps, or if we need to explore further.