how do I install GetSimple on OSX snow leopard?

[hameau]

Why is that happening?

Almost certainly a permissions problem in the GS base directory. From your phpinfo, the webserver is running as user _www, and this user must have write permissions in that base directory.

Several ways to achieve that, but for a local test server I suggest setting the GS base directory group to _www, user to your own user account (this is probably already set) and permissions to 0775. That will allow both you (so you can make changes to, for example, gsconfig.php) and the webserver to have access to the directory.

This is based on Linux use, but should be applicable to OS X.