How to configure postgresql in Lion
First up, what's the error you're getting, and how are you getting it? Are you trying to re-run the installer?

In answer to two, take a look at http://www.jonathandean.com/2011/08/postgresql-8-4-on-mac-os-x-10-7-lion/ who seems to have gotten it sorted.

There absolutely is a GUI for PostgreSQL, it's called pgAdmin 3 (or pgAdmin III). If you successfully installed the OS X version of PostgreSQL from their site, then you should already have it (take a peek in Launchpad). If it's not there, you can get it at http://www.pgadmin.org/ but read the downloads page carefully---it isn't well sorted and the most up-to-date version is in the middle of the list.

As EmmEff said, in future Homebrew or MacPorts might be easier (especially if you like playing around!) since you can wipe the install cleanly.