The ARM profile is subordinate to and tied to the radio profile. Create a new Radio Profile using Configuration> Advanced Services> All Profiles> RF Management> 802.11a (or 802.11g) radio>. Type the name of the new radio profile and click on Add. To back to All Profiles> RF Management> ARM Profile, type the name of a new profile you would create and type Add. Go back to All profiles> RF Management> 802.11a or g radio and find your radio profile. Assign your created ARM profile to that radio profile.
Go to Configuration> Wireless > AP Configuration. Click on the AP Group you want to assign the radio profile to. Expand RF Management. Click on 802.11a or 802.11g radio profile. Change the dropdown to the right to the radio profile you created above. That will assign the radio profile and then the ARM profile to the AP group that you want.
I hope this helps.