Have you tried getting your friend to connect to your IP directly?
Have you tried Port Forwarding?
Try disabling your firewall.
P.S. Your image seems to be broken.
From what I can tell, this mod replaces San Andreas' map to what is called 'Myriad Islands'. The only current way to do this is to script the models and load them client-side but the download would be far too large and MTA's streamer wouldn't handle it. MTA cannot replace SA's map either. This was already suggested in the past with GTA: United, it is not currently possible but could happen some time in the future.
@Arran
Seven months is nothing, I guess you weren't here for the Developer Previews or 1.0 itself.
I'm all for bug-related issues being added to the roadmap of 1.1, the release should be as stable as possible(not to repeat what happened with 1.0). It's new features that increase the workload.
Use THIS batch race map converter by arc_ to convert your race maps into the required format for 1.0.x. After converting place your maps in C:\Program Files\MTA San Andreas\server\mods\deathmatch\resources\.
To disable compatibility settings manually for a program, right-click the icon for the program, click Properties, and then click the Compatibility tab and turn it OFF for both gta_sa.exe and multi theft auto.exe.