The earliest I have found that you can unlock a helicopter is during the main storyline. Once you complete the mission Trojan Whores, you receive a Vulture helicopter for the mission reward. It's accessible at the Saints HQ pad in Sunset Park. Just go outside next to the pool (after you're inside the HQ, so the outside of the penthouse) and you'll see the floating key indicating a garage, which is where you'll be able to store future helicopters.
I was level 15 when I acquired this, which is 9 levels earlier than you can receive the Heli Homie. I'd also done a bunch of activities on the side to raise my level though, so if you just go straight for the Trojan Whores mission, you'll unlock it earlier.
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.
Best Answer
You essentially just need pedestrians walking on the sidewalk - the more, the better.
There's not a particularly populated area I can suggest in the game, just drive around a bit in downtown, and find a spot where people are walking around.