When used with a nickname, /dcc send nick c:\file.ext works fine, because mIRC knows how to be in touch with the given nick, and tell him "hey, I want to send you a file".
Now if you want to use an IP address, you have to specify both IP *and* port. mIRC's help file quote :
Note: If you want to connect to someone else's DCC Server you can specify an IP Address and port instead of the nickname, eg. /dcc send ipaddress:port
It actually works only if the person on the other side is running a DCC send server (in mIRC's options)