Sound's like option 1 then (which does refresh on login).
While option 2 seems easier to subscribe-they'll have a harder time remembering to reload.
And as far as directions go-words, pics, and a walkthrough flash/wink are about all you can do. I think the best thing you'll find is those who 'get it' quickly help those that don't-at least of my experiences with college faculty/staff & students.
You could play with-in some kina batch way-ie they check a box on a portal page and it runs a script on your zimbra box:
(sry my syntax is a little fuzzy but you get the idea)
Another cool thing you might do-also encourage people subscribe to an RSS feed of the cal in their folders area.
mbox> adminAuthenticate -u https://server.domain.com:7071 email@example.com password
mbox> selectMailbox firstname.lastname@example.org
mbox email@example.com> createMountpoint --view appointment /athletic-calendar firstname.lastname@example.org /Calendar
Even more traffic-but you could force out a folder called 'feeds' and put a rss of all the cal's in it...