I ran into the same problem when installing Moodle on Centos 5. I had created the moodle data directory in the usual place, ensured it was writeable by my apache user but I kept getting the message that the installer couldn't find it.
The solution was simple but not obvious.
In the previous screen where the installer performs the Php checks it said I had safe mode on. This meant Php couldn't write to any directories so couldn't write to my moodledata dir.
In my /etc/php.ini I changed the following line -
safeMode=On
to -
safeMode=Off
and re ran the installer. All worked and install fine.
John