Normal installations will create a registry entry which you can look for. You find the entry and grab the install directory. Then...
During your install.
1. Present a dialog to the user with the default product path (or the found one above.)
2. The user can modify the path.
3. When the user says ok you then verify that the correct dll is at the path specified.
4. If the dll (and correct version) is not found then put out an error message and go back to step 1.