Error installing themes / asked for FTP credentials

Hi

when I try to add or remove themes I cannot do so because I am asked for FTP-credentials although the installation is on a local machine using MAMP.

It does not seem to be an error related to file permissions as I set the permissions to 777 in MAMP and also checked them in the console – they are all set correct and recursively.

So as I am using the WordPress skeleton from Mark Jaquith (content and wp-system divided) I wonder whether I did something wrong in the setup process (e.g. in the local-config.php)

What is the best way to investigate into the issue?
I have to somehow try to find out the location where wordpress tries to add or remove a theme so that I can check that this location is really the correct one.

Your help is highly appreciated.

Best regards
Marc