If you just downloaded it from STEAM, it should already be patched to the latest version with the Realism V1.07 patch already in it.
Here's a site I found with a bunch of custom missions for 688i HK
http://www.geocities.com/SiliconVall...8missions.html
688i can also only handle a certain number of scenarios in it's scenario folder or else it won't be able to handle it and crash (I forgot how many it can handle). When you reach that point, you can either manually swap scenarios out, or use the Scenario Manager you can get here which seems to work kind of like our friend "The Generic Mod Enabler" (which I imagine you could probably also use for it).
http://www.lrim.com/scmgmt/help/toc.html