I have installed OpenWRT successfully on my WRT54GS (1.0, I think). That is: I can telnet to the device and play around. In the original Linksys software I had configured my LAN access as well as my WAN-access (thru PPPoE). However, now my computer cannot connect to the internet.

I actually thought that the original configuration was saved by OpenWRT and used automatically when starting freshly with OpenWRT. Apparently this is not the case - at least not with PPPoE (?). How do I get PPPoE installed and working?

I have found some information on PPPoE. What I don't understand is this: I have installed the latest snapshot. Does this mean I have no PPPoE installed? And if that is the case, how am I supposed to install PPPoE with ipkg without an internet connection?

You can download the ipkg and the file packages to your local webserver. Then you may drop connection to internet and add your local webserver to ipkg.conf.
Then you can install the pppoe (i work with pppoecd) from your local webserver with ipkg.
Or download it to your local ftp-server and "wget" can bring it to your router.

A good description is at:

http://www.freifunk.net/wiki/FreifunkFirmware   (German)
http://www.freifunk.net/wiki/FreifunkFirmwareEnglish    (English)

In the beginning openwrt is not easy, but if you did some practice you will love it because you will find a new feature or new idea what to do with it everyday.

Ok, sure - it's not that I haven't done something like this before but I am totally new to this device. Thanks very much for your hints.
May be a stupid question: Can I use the Freifunk pppoe package with the ordinary OpenWRT snapshot?

I didn't try it, but i think it will work.
Or try the freifunk-firmware itself.
It works for me very good. You might not need olsr (i need it), but dropbear i think is essential.