In the main SH3 folder, where GWX was installed into, there is an icon, of something like a wheel, but it's iron and jegged (sorry, I forgot the english and slovene word
). Next to it, it says JSGME. There is also a MOD folder in the main SH3 folder. This is where you get the 23rd December update into. Then you run JSGME, pick the mod on the left part of the screen (screen of JSGME, there will be 2 collums), then click on the > and it will install the update!
Welcome aboard!