If you are registered to vote and get in line at the polling place before the polls close, you ARE allowed to vote, regardless of what you may be told. If you aren't allowed into the voting booth, ask for a provisional ballot. In some states, you can vote even if you're not yet registered. When in doubt, go to the polling place, and bring ID. (The best ID you have, even if it's just a utility bill.)
And if you are eligible to vote and don't, then as is my long-standing policy, I will not speak to you at all on Election Day. Perhaps longer. Voting in this election may be the most important thing you can ever do as a citizen of this country.
EDIT: If the above website does not work for you and you live in Pennsylvania, go to http://www.dos.state.pa.us/voting/cwp/view.asp?a=1195&q=443089&votingNav=| , click on your county, and contact them to find out where you're registered. Or in Philadelphia, call 215-686-1505 or 215-557-3600 or e-mail email@example.com. Again, I'll help you with this if you need me to.