No...as this question is oft asked...the answer is still the same. The e53 X5 does not require "battery registration" for a replacement battery.
Whoever spoke to you is thinking of the new BMWs (like the e70 X5) that uses the MOST communication bus system (i-Drive). Your e53 X5 is just like all BMWs prior to i-Drive and you have no need to "register" a replacement battery.
Make sure you didn't blow a fuse when you replaced the battery by checking ALL of the fuses for the central locking system (front glove box & rear cargo area).
You can check your red LED status light (the clown nose under the rear view mirror) to see how it flashes when central locking is activated (you'll have to manually insert the key into the driver's door since the keyless system is not functioning).
The remote key contains 3 features that are all separate but work in conjunction with each other:
- EWS - immobilizing drive-a-way-protection system
- FZV - keyless entry/central locking system
- DWA = anti theft alarm siren system
Again, if you arm the DWA system, you can check the red LED status light to see if all monitors are working correctly...if not, find the monitor that is not working and repair it (or disconnect its electrical harness) and that may restore the keyless function to your FZV system.