If it's anything like my SL, there is a procedure in the owner's manual to reinitialize the keyless remote once the battery in the remote has been replaced.
You have to put the key in the ignition tumbler and turn it to the I or II position to synchronize the remote with the car's computer. After that, the remote will be able to activate the keyless entry.
If that STILL doesn't work, then the remote must be replaced ($$$)
. I have one good and one bad remote, so it's possible...