I've been on the Strip before.... but now when I try to Enter the Strip it tells me a key is required. Where's the key?

Use the monorail at fort mccaran (wear a ncr outfit to get past the guards). That should work.

I'll try that. Thank you.

i have the same problem, but now i cant enter via monorail, because the monorail exploded when i did the mision "i spell a curse on you", any other way i can enter the strip?

I believe that the Securitrons have keys on them. Might have to kill one since I don't think you can pickpocket them.

Thank you, now i can continue with my quest.

Could always cheat if you're on the PC by opening the console, clicking the gate and typing "unlock"