If you go to Project > Add track > Instrument track...Is Spectrasonics Atmosphere listed there?
If it is, can you select it and will an instrument track appear on your mixer and tracks window? Then you need to assign a midi track output to that Atmosphere instrument track.
If it is NOT listed in the Project > Add track > Instrument track location...got to your MOTU Audio Units cache and put it in the trash. Delete the trash, reboot and the next time you start Performer...it will rescan for your audio units and find the "Atmosphere AU.component" in the Library > Audio > Plug-ins > Components folder.
And also if you have put the Atmostphere sound library .dat file in another location other than that same folder, you need to have an Alias of the "Atmostphere.dat" file in the Library > Audio > Plug-ins > Components folder as well.
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