I would recommend just installing SIMBL without checking. These days, because InputManagers can only go in /Library (and not ~/Library), SIMBL is always in the same place. In this case, installing SIMBL can't really hurt -- the author doesn't update SIMBL often at all these days, so it's very unlikely that you'll accidentally "downgrade" a user's SIMBL installation. (The last release of SIMBL was in 2006.)
To add SIMBL to your project, you can choose "Add Package Reference" from the "gear" button in the lower-left. You can also just drag the SIMBL folder (from InputManagers) into a new choice (see "New Choice" under the same gear button) and set its install location manually.
As an example, here is a .pmdoc file for a SIMBL plugin I wrote called Scalp.