Which version of mIRC are you using?
Are you behind a router or something? If yes, you should ask someone to forward the DCC ports (usually 1024-5000) to your machine.
Do you know other people using the same Internet Service Provider as you use, and do they have the same problem? If yes, it is probably a problem of your ISP and we usually can't help you with it.
Maybe the solution is in this thread