AFAIK, there is no map named "sedna" yet. It is sedna-betaX where X is whatever beta it is. Maybe there are also sedna-alphaXs lying around, but I've not seen 'em.
If you are unsure of which version you have, you can extract (or peek at the contents of) the PK3 (it is just a zip) and look in the maps directory for sedna*.bsp.
Alternatively, type
/map senda-[TAB]
on the in game console