[OSM-newbies] josm and layers

[graham]

I'm having problems with the new josm, but coincidentally have also just 
got a new computer, and don't know which is causing the issues.

I can upload my files ok, and also download the osm data, or the yahoo 
imagery. What I can't do is see more than one layer at a time. The 
'merge' button is permanently greyed out, if that's the way you're 
supposed to do it. If I make a new layer visible it toggles all other 
layers off. So I can't see my waypoints at the same time as  my tracks, 
or either at the same time as the existing osm data, which pretty much 
stops me doing anything. I'm wondering if there's something as simple as 
a 'make layer background transparent' option, but can't see it.

I'm running on debian testing (lenny) with the sun 1.5 jdk. There are a 
few exceptions being generated, but I think they're all minor and plugin 
related.

Any suggestions?

Thanks
Graham