Take the bike to a shop and first ask for them to flex-hone your seat-tube. Its basically a cylidrical wire brush with ceramic impregnated balls on the ends. This should remove any burrs that may exist.
Try that, if it does work great, clean out the seat tube well, grease and install.
If not, have them ream it. They should have an adjustable reamer (most shops have one set up for 27.2 since that is the most common size for road bikes) and see if it will drop it (while not turning it).... if its WAY too tight to drop in, you may not have a 27.2 seat-tube. Reaming is not really designed to increase the hole diameter as much as its used to knock off blow-through slag and MINOR inconsistancies in the frame.
When I used to build bikes from frames up, I would ALWASY face and chase BB's, face headtubes, and ream seat-tubes. Lots of bikes need it and you would be supprised at how many higher end bikes come with all three items lacking.