If you are still having problems passing values correctly into your dll, you might want to check out this link that details what the various equivalent C data types are. Otherwise, I would recommend going to the shipping examples that come with TestStand for extra assistance. The example that I think would help the most can generally be found at <C:\Users\Public\Documents\National Instruments\TestStand 2014 (32-bit)\Examples\Fundamentals\Passing Structs to Code Modules\C\Passing Structs to Code Modules.seq>.
Thanks for the pointers. I have seen (and used) this link before and started everything based on this example. Did you happen to make any progress otherwise?
Since we are currently working on this issue in another Service Request, could we try to keep our progress in one spot? Whenever we reach a conclusion, I'm sure that others would appreciate your posting our answer here.