The DCCSERVER method they're using at port 59 works in a way that requires the receiver to have the port forwarded, instead of the sender. In mirc_options/connect/options/advanced, you leave your dcc port range the way it was before, because dccserver doesn't use *that* dcc range. Instead, if you have been able to dcc send to others using the normal dcc method, ie "/dcc send othernick filename", then after doing the "/dccserver +sc on 59" that they tell you to do, all you should need to do is configure your router/firewall to enable port 59 the same way you're also enabling ports 5000 thru 5004.
As an aside, if they're using 59 because they aren't able to configure their own dcc ports, it might be easier for them to use the passive dcc send feature that was added in mIRC 6.17. It still requires the receiver to have open dcc send ports, but it would allow you to receive within your 5000-5004 range without configuring port 59.
