actually, with a mech like the Nemesis, you can put the battery in either way and it will fire. However, it is recommended to have the positive up and the neg on the button....so you're good there.
Take the top cap off the mod and screw on the atty. There will be a thumb screw under the 510...make sure it is in contact with the atty. Make sure the kick ring is not installed. It is a thin (3mm) body tube section. Screw the mod together and tighten up the switch until it gets snug. You should have zero battery rattle when you shake the mod.
Nemesis advice.....I feel like I just went back in time! Great mod and Welcome to ECF. It should be said that when using a mech, please use good batteries from a reputable vendor. Samsung 25R are the ones I trust. Illumn has them for about $6 each. There are others, Sony VTC 4 or 5, LG HE 2,4 .... read up on battery safety and Ohms law. Plenty of information right here in the forum.