Thanks to the FAQ, and those forums.
After having successfully installed Geeklog on my Mac, I wanted to be able to view this site from my PC (I have some perso. theme to adapt) ; the config file was setted with an IP adress :
$_CONF['site_url'] = 'http://192.168.1.2/blog/etc...'
Waoo ! Prof. Theme looks splendid on the PC.
I play a little with the PC, log into with the Admin access, and everything is allright.
Then, I come back to my Mac, and ...no way to get the Admin menu !!!
Deleting caches, cookies, re-initializating browsers... nothing helps : still no Admin menu.
Mince alors !
Changing
$_CONF['site_url'] = 'http://192.168.1.2/blog/etc...'
to
$_CONF['site_url'] = 'http://localhost/blog/etc...' makes this menu reappear ! Youpi ! (even if from the PC, all pathes are wrong, now)
...very strange, isn'it ?