MAMP security issues

Hello,
I am running wordpress through mamp on an imac. There is always a warning that you should run apache as www/mysql, rather than your own user name when connected to the internet, however when i try to do this, that user doesn’t have permission to write to files (eg the uploads folder for pics etc).

When I try to specify www as a user to give it permissions, it says that username already exists (although it never appears for me to select). Is there another name i should be using?

Forgot to mention – I’m using the Lion operating system.
Thanks in advance,
Tim