I've never used a T3i, but the battery latch is more or less the same. You should just slide it to the side and the battery will pop out. You can turn it over while holding it to the side to help it.
However, do you by chance have a generic battery you bought off a third party? Many of them are sized incorrectly and get stuck. If that's the case you need someone to hold the latch open while you use a good pair of needle nose and try to get purchase on that little plastic tab. I had a couple like that for my old 450D. I ended up wrapping dental floss around the batteries so I had something to pull up on.
