I am not entirely sure if it fits here, but "NotImplementedException" looks like a missing feature.
I can't run JDownloader with IKVM. Basically, I call \bin\ikvm.exe -jar JDownloader.jar, get a large console output (attached) and it stays like that for an hour or so. IKVM doesn't close nor does it advance, I just get the same image on the console for whatever how long I keep my console open. Nothing else happens.
I run Windows 7 x86-32 with .NET 2.0 SDK installed (as mentioned at the requirements page). All .NET framework updates are installed. WU just shows an empty list. IKVM itself works, I can call other JARs (e.g. JDownloader's updater), but JDownloader doesn't work.