There are two main venues for getting support for OpenMM: the discussion forum and the Github repository. There is some overlap between the two, but generally speaking the forum is for user oriented issues while the repository is for developer oriented issues. If you have a question about how to use OpenMM (including writing programs that access it through its public API), post on the forum. If you want to suggest a change to the code, or if you think you have found a bug, open an issue on Github. The core developers monitor both, so don't worry if you aren't sure which one is most appropriate for your question. We will see it either way.
You also may want to consult the documentation. It is quite thorough, and you may be able to find the answer to your question.