when I try to equip the RC Possessor in Saints Row the Third, the weapon is not selectable - why is this? How can I use this weapon?
You don't specify where you're trying to equip it, so I'll give you a broad answer that should cover most scenarios.
You can't equip it at all until you own it. The RC Possessor requires that you have completed the mission "The Belgian Problem." An easy way to check is to see if you have Oleg on your phone list as a homie. If you don't have Oleg, you don't have the RC Possessor.
If you can't select it from the weapon wheel, it might be out of ammo. Visiting any Friendly Fire will allow you to stock up on RC Possessor ammo.
In some situations (certain missions and activities, for example) you will be locked out of your weapons or given a particular set of weapons you must use. In these cases, you won't be able to use your other weapons, even if they're unlocked and stocked with ammo.
If there's some particular situation you're in where you can't use it, add those details to your question and I'll try to address those specifically.