mIRC uses a protocol called 'DCC' - this means 'Direct Client to Client'. A direct connection is established between you and someone else, meaning no information passes through the server itself.
SendingTo send a file to someone, simply use
/dcc send nickname file.name.here - Only specify the "file.name.here" if the file you're sending is in your mIRC directory (the folder with the mirc.exe in it). To see your mIRC directory, type
//echo -a $mircdir.
You can just type
/dcc send nickname and then choose a file to send from the dialog box that appears. Once a file is double clicked, the transfer will begin.
Many issues can be experienced when trying to send a file. Most of those issues are covered in
this thread. It's also a good idea to use the Forum's
Search feature and see if you can find help with that.
mIRC.org also have a good site referring to DCC technical issues.
ReceivingIf you have issues receiving a file the chances are the problem is coming from the person that is sending you the file, rather than your end being the issue. It's a good idea to refer who ever is sending you a file to the links above to see if it is them. Some of the most common problems include an IP mismatch (use
/localinfo -u) and the XP Firewall - see
here for instructions on how to setup the XP Firewall with mIRCs IdentD and DCC features.
If you're sure that it's your end that is the problem, it could still be an IP mismatch at your end, in which case you need to type /localinfo -u instead. Other suggestions can be found on
this page and of course with the search feature.
When using the search feature, expanding to 'All Forums' and 'All Posts' is a good idea to obtain the best results.
What error message are you receiving when trying to send a file? Unable to connect perhaps? This would imply it is the sender's end that is the issue. Try receiving from several different people.
Hope this helps a little

Regards,