Multi server administration - Failed to import Maintenance Plan to named instance
Your problem is that the Maintenance Plans are really just SSIS Packages under the hood. As your MSX server is 2014, I'm assuming that is where you created them, which means the SSIS package version will be 2014. You will most likely have to downgrade your packages to have them run on SQL 2008 R2, which means you won't be able to include them in your master job server.

This link states that SSIS only supports target servers that are the same version or higher than the master server version, so your 2008 R2 server would either have to be the master or you need to revert to using T-SQL based maintenance jobs instead of Maintenance Plans.