NAS folder not found

hi,

following problem:

i've got a Mac Book Pro (MBP) running on Lion and a IMac running on Snow Leopard.

Both Computers are connected to the same wireless network.

I've attatched an external hard drive the the wireless router with all my media on it.

Now, the external harddrive appears on the IMac's finder in the "shared" section, along with Mac Book Pro (i've activated file sharing between the two computers). However, the extenal hard drive does not appear on the MBP, whereas the shared folder of the IMac does. I really want to acess with both of my computers the external hard drive, so does anyone have any idea what i have to do in order to get it working with my MBP?

BTW. firewall is off, so its not a question of blocked incoming connections.

Thx in advance