When you sign up to the geocaching website (that tells you where all the caches are) you get a free, basic membership, but some people choose to pay to upgrade to a premium account which lets them access extra caches and website features So to be able to download the details for the premium caches you need to be a paid up member.
Thats true, you can't access the page, so wouldn't be able to log the finds. But it might give a good intro into caching, and the actual finding of the caches. Think you'd have to drive to do a few non-premium ones
you can log premium caches if you arent a premium member. go to the cache on the map, zoom out to the point where the premium caches show (they disappear for me if i zoom in too far) click on the cache, and it will bring up the normal speech bubble thing, it wont let you go to the cache page but if you click the little 'log visit' link at bottom right, it lets you log them. hope this makes sense