Our MBS Plugin works fine so far with the FileMaker 13 version. Even WebDirect with plugins works fine on Mac with the 64bit enabled plugin. For Windows, the 64bit version is not yet ready. If you see an issue, please report as soon as possible.
and be aware that FM 13 can work fine with FM 12 databases, just if you create a new database, you may want to create it in FM 12 so you are able to open it in FM 12 and FM 13: