Installing on ubuntu should be quite easy.
i would reference here a german ubuntu wiki. https://wiki.ubuntuusers.de/Spiele/Return_to_the_Roots/
You should need the following packages:
libsdl1.2debian libsdl-mixer1.2 gettext libsndfile1 libsamplerate0
Afterwards download the nightly build and put your S2Gold Installation into it, like you did on windows.
Check out the lobby and/or the irc channel #siedler2.5 on irc.freenode.net
The irc Channel has a lobby connection, so you can chat with ppl logged into the lobby and see if someone logs in.