I just got into the viggen and is learning the nav system, i know howto do a nav fix using landmarks, but my question is, how do i do it when there are no landmarks too be seen, like when flying over the sea??

Was flying the "hunting the hunters" mission and wanted to use ils when landing, but the L1 waypoint had moved away from the airport and into the sea, so the ils was leading me into a very wet landing. Now it wasnt a problem because the weather was nice, but if there had been bad weather it would be a problem...

You don't
Always set one steerpoint at a recognizable coastline spot, easy to identify on the ground radar, and use that to do a radar fix.

omg, now i feel really stupid, lol. My brain seem to having some issues today. Thanks

haha please don't.
You're obviously in explorer mode - keep that going. Best way to learn new stuff.
Question everything.

I have another small question and dont want to start a new thread... I cant seem to get any weapons loaded in the viggen in DCS 2.0.5 Nevada, is it a known bug or something on my end?? Would be nice to learning too use the Viggen with better scenery but it just doesnt work for me....

This is a great idea, and I always do feet wet/feet dry waypoints whenever possible to update nav before and after overwater flight. Even if you don't have a specific part of the coastline you can mark, if you come in perpendicular to the coast you can at least update the "speed line" component of your position (i.e. distance from your aircraft to the waypoint).

Another thing I like to do is make my first waypoint a fix waypoint (preferably a visual fix) as soon as possible after takeoff. Since the nav system puts your present position at takeoff (weight off wheels) at the center of the runway, you may have a nav error of 1km or so depending on the length of the runway and the length of your takeoff roll. A visual fix (ideally along runway heading) is great to tighten the nav before starting your route.