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1. Delete the existing postgres user with the following command:

   Code Block
   sudo dscl . delete /users/postgres

2. Recreate postgres user in the system preferences (name it postgres, and preferably chose the postgres123 password)
3. Download and install Postgres 8.4 as you would normally do (http://www.postgresql.org/download/macosx) 

Alternate installation if you have trouble building you can find here.