With SqueezeCenter (the latest version of Slimserver) you need to connect to the SqueezeCenter itself as it is the management console, via
http://192.168.80.1:9000 I think from memory... it is installed on your core, the core talks to it, and it talks to your squeeze devices. You can update the database from in there... but do note, that if you use the squeeze devices to choose songs, etc, LMCE won't know this and you will have some difficulties controlling them afterwards (like moving a song from device to device, as it isn't aware of that song, nor has a stream been set up for it)