Your question is so general as to indicate that you have no idea what
you are doing.
You need to figure out which 3rd party firmware you're using, go to
the site, determine which version works with your router. Which
version of the wrt54g do you have?
Then ask the people that work with that firmware on that forum, why
release xxx.xxx won't install on wrt54g version X, even though you did
X.
Yes, DD-WRT is recommended. You should read up on what you are doing
and hope you didn't brick your router by doing it the "easy" way !