I'm a bit confused...
There is no world map from OSM available in KAP format (pretty understandable as it would be of very little use)
The KAP download facility works fine for me in Firefox - remember that you have to select a chart you want to download to be able to download it (see the screenshot)
I don't understand at all the relation between KAPs, OpenSeeMap format (what is it, BTW?) and the Garmin charts
link to the wiki, sorry. The coverage of the OSM KAPs is apparently not the same as the coverage of the Garmin charts
generated from the OSM data.