I presume you are trying the live cd.
At the login screen choose demo as user with password demo.
You could also use root with password as root.
When/if you install you click on install icon and follow the instructions.
I would advise a bit of reading beforehand.
Try the mepislovers wiki
http://www.mepislovers-wiki.org/inde...itle=Main_Page
and the Mepis users guide (in pdf format)
http://www.mepis.org/files/MEPIS%20User%20Guide.pdf