Ok, so you've got Crysis installed and patched.
You've installed the Mod.
Time to fire up Crysis.
When all the bla bla is finished, you start on this screen.
Click on the MODS button and it takes you to here.
Click load and you will be greeted with this screen.
Click yes and the game restarts........ Ooooo Yeahhh!!!
Once in MWLL your first screeny is this...
Yep, you click Muliplayer..... and go to this...
When you click on Internet game, you will be required to enter your Gamespy ID and Psswd.
Didn't work with mine, fortunately there's an option to create a new account (very easy and straightforward)
Once the signing in process is complete..... finally you get to the server screen.
The rest is up to you 
Crysis works in both 64 and 32 bit mode.
Tried both and they seem to work fine.
If you want to use 64 bit with a downloaded game, you need to use the launch64tool for secuROM which is located
on my HDD in..... C:\Program Files (x86)\Electronic Arts\Crytek\Crysis\Bin64 yours will differ, but you get the idea 