I would check the remotes first. Make sure the batteries are good, and make sure that the little light in the remote blinks when you press a button. Keep in mind that this light may still blink even if the batteries are too weak to operate the remote properly. Additionally, from what I know about the Rattler 300, the remotes should have green lights on them. If they're red, then they're probably the wrong remotes.
Now, if you've checked everything and you think the remotes are good, but they still don't work, then you can try coding the remote to the alarm. The Rattler should have a valet/push button under the dash somewhere. You'll need to use this button to program the remotes. Additionally, when you program the remotes, a car door has to be open.
To program a remote, turn the key to the ignition/run position (but don't start the engine). Then, tap the valet button once, then hit it again and hold it down. While holding the valet button down, press button [1] on the remote. You should hear a chirp. If not, then just stop and turn the key back off -- something is still wrong somewhere.
If you did hear the chirp, then let go of the valet button, tap it again, and then hit and hold it down again. While holding down the valet button, hit button [2] on the remote, and you should hear two chirps. If you were only using buttons [1] and [2] before, you can turn the key off and stop. But if you were using channel 3 (buttons [1] and [2] held down together) for some particular feature, then go to the next step.
Tap the valet button again, then hit and hold it down again. While holding the valet button down, hit both remote buttons together ([1] and [2]), and you should hear 3 chirps. At that point, you should be able to turn the key off and be done with the programming, and your remote should work.
If you couldn't get it to program at all, then there could be any of several causes. The door trigger wire of the alarm may not be hooked up properly, causing the alarm to not see that the door is open. It could still be the wrong remote. Or the programming features of the alarm may have been disabled (I think that on this alarm, you can open a trap door on the brain and move a jumper to disable programming features). At that point, it requires digging deeper to find the problem. I hope this helps!