Try to clean your solution and then build (or rebuild). You may need to clean your registry. Please note that usually you do not need to do this - just rebuild enough but sometimes the registry may be too messy so SolidWorks 'confused' and doesn't load correct version. You may also just change the add-in GUID so SolidWorks 'thinks' that this is completely new add-in.
Regards, Artem Taturevych | Snr. Developer | IC3D ANZ
IC3DSteel – New Steel Solution for SolidWorks