When you get enough renown in the German campaign you will eventually be able to get the sub. However, if you would rather take it for a spin now you can try some missions I put together for the German side of things. I hate tooting my own horn, but you can check the thread out here.
There are single missions and war patrols included. One war patrol uses the Type XVIII and a few single missions use the XVIII.
http://www.subsim.com/radioroom/showthread.php?t=135971