You can back up your tables in two ways (I suggest using both, just in case).
First Way (the standard way)
At the command prompt type:
[amras:~/Desktop]% sudo mysqldump -c -v --all-databases > dump.sql
... or ...
[amras:~/Desktop]% sudo mysqldump -c -v database_name [table_names] > dump.sql
Without the redirection operator it will dump it to stdout--the screen. Now pico dump.sql and have a look through it to make sure everything you need is there. You should see some CREATE DATABASE, CREATE TABLE, and INSERT INTO table VALUES statements, to compeletely re-create your database structure and data.
Then to restore the data:
[amras:~/Desktop]% mysql -u root -p < dump.sql
You will need to use the mysql root user to reload data unless you have already configured your local user to have privileges to create databases and stuff...
Also, make sure that if you did a dump with --
all-databases you didn't also dump a blank "mysql" database which will overwrite your new properly configured one
Second Way (not 100% sure about this one)
Note: You'll have to "sudo -s" to get access to these folders.
Your databases are stored in
/usr/local/mysql/data/database_name/tables[.MYD][.MYI][.frm].
Just copy the files out of the data/ folder to somewhere else, and then copy them back. I'm 90% sure this way will work, the 10% discrepancy is because I'm not certain that there isn't some file that has to be updated to point to the database files, or if MySQL just reads all the appropriate files from that folder.
NOTE: If the first way works properly to re-create your data, don't restore with the second method!
You might also consider upgrading to MySQL 4, thay've now released native binaries for OS X/Darwin (no more entropy.ch):
http://www.mysql.com/downloads/mysql-4.0.html