if it looks like this underneath
http://www.irisvista....ove-hard-drive-01.jpg
then the panel that covers the fan is the one in the top right with the two round bits on it. it's held on with 3 screws by the looks, so removing those screws should give you access to the fan and the heat sink. and it should look like this inside.
http://www.irisvista....teP15-heatsink-03.jpg
If you remove the three screws circled in red in the above picture the fan will lift out, you can then get in there with something like a soft make-up brush and try and carefully get all the fluff out from the fan area and the heat sink,the one in the pic above is pretty fluffy! they should be nice and clean so air can flow through them.