I also have a fluval 1 plus - it does come apart, quite easily!
It pulls into two pieces, just around where the top suction cup is. Grab it at both ends and pull, it'll come into two parts, and you'll be able to get to the sponge inside (just pulls out).
While you have the sponge out, I'd recommend cutting it in half (lengthwise) to give you two seperate panels - this way, you can put any donated media in between the two sponges. This also means that in the future, you can clean/swap/donate half of your filter sponge and replace it with new, leaving the other half (complete with bacteria colony) in place.