I need to go to Canada for work. I have a dropped arrest for being with people that were stealing bush campaign signs.

I have an arrest for election fraud for being with people that were stealing signs. The charge was dropped and supposed to be removed from my record. I was stopped at the border when trying to cross for work. They let me through and said I need some kind of paper maybe? What do I need to do? I have to go there for work and I don't understand why it would be a problem if it was dropped.