I suggest describing step by step exactly what you are doing and exactly where in this process you get that error message. That might at help us better guess where there might be a problem.
I have a FMP file called Schedule Planner. It is one database with 4 tables that are all related to a serial number. The file is currently not open. I went to Tools>Developer Utilities. I added the Schedule Planner file, created a new folder and only checked the "Create Runtime solution application" in the Specify Solution Options window (screen shot attached). After clicking "Create," I get the "This file is locked or in use." message. On the Developer Utilities window, it shows Status: Copying Solution Files..... Internal Runtime Libraries info.plist.
I'm running Mac OS X 10.10.3