Copy add-in dll file to one of following locations:
a) Anywhere, then *.addin file <Assembly> setting should be updated to the full path including the dll name
b) Inventor <InstallPath>\bin\ folder, then *.addin file <Assembly> setting should be the dll name only: <AddInName>.dll
c) Inventor <InstallPath>\bin\XX folder, then *.addin file <Assembly> setting shoule be a relative path: XX\<AddInName>.dll
3) Copy.addin manifest file to one of following locations:
a) Inventor Version Dependent
Windows XP:
C:\Documents and Settings\All Users\Application Data\Autodesk\Inventor 2012\Addins\
Windows7/Vista:
C:\ProgramData\Autodesk\Inventor 2012\Addins\
b) Inventor Version Independent
Windows XP:
C:\Documents and Settings\All Users\Application Data\Autodesk\Inventor Addins\
Windows7/Vista:
C:\ProgramData\Autodesk\Inventor Addins\
c) Per User Override
Windows XP:
C:\Documents and Settings\<user>\Application Data\Autodesk\Inventor 2012\Addins\
Windows7/Vista:
C:\Users\<user>\AppData\Roaming\Autodesk\Inventor 2012\Addins\
4) Startup Inventor, the AddIn should be loaded
To unregister the AddIn, remove the Autodesk.<AddInName>.Inventor.addin from above mentioned .addin manifest file locations directly.