I bought new distributor why won't it go back in i can't get it to go back down

IF it's the correct distributor (?) Hopefully you made reference marks before removing old distributor (?) .. *After you get it lined up properly* (the exact same way the old one came out) turn engine a 'tiny' bit so it will drop onto the hexagonal oil pump shaft that's down in the engine.... Look at the distributor shaft to see the female hex socket... Ignition timing will need to be set per instructions on decal under hood.