I did this once before... I got the demons heart, now all I have to do is get to the core to place it! But where is the door? I simply forgot it, and the door in same room as Tolgerias was hangin' out at, it's locked!!! :confused:

You have to place the heart in the powercore.
But since you've done this before there might be some other problem? You can check This guide (http://www.sorcerers.net/Games/BG2/Walkthrough/sphere.htm) here at SP if it is no help at all. Just scroll down to where you are stuck.

If you've beaten Tolgerias, then you just need to go from that room through the fire room or the ice room to the core. The locked door is (IIRC) just a storeroom that you cannot access anyway, so don't bother.