Hi mrjynx, a couple of different things here - for doing an edge fillet where you want to round off a particular edge that edge should be a joined edge where 2 surfaces are glued together.

In your attached file, your surfaces are separate objects that have not been joined together yet, so select those 2 pieces and run Edit > Join so that they have a joined edge in common and then that will be filletable if the edge is selected.

And then the other thing is to make sure you get the actual _edge_ selected and not your original generator curve objects which are still sitting around right in the same spot as the edge you want to fillet. Curves have selection priority over edges, so it will be easy for those curves to get selected instead of the surface edges. To avoid this, hide or delete those original curves (you can use the scene browser, Types > Curves section to hide all curves with one click) before trying to select the edge and that will help too.

So get those surfaces joined, and then get those original curves hidden, then you'll be able to do a drill-in click (that's where you do a second click on an object, the first click selects the whole object and then a second click selects sub objects like faces or edges) to select the edge and then fillet it.

You may want to finish off the other pieces of your object first before doing the filleting though, if you want to fillet other pieces coming off there too it's usually better to do it all in one shot so that you get corner patch generation for all junctures, you can get some problems if you only do a little bit of filleting at a time in areas where multiple fillets are running into one another since you will be creating more little edges and things like that on each separate fillet pass.


Check out here for some links to discussions and general tips for people who are coming from a sub-d / poly modeling background:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4865.2

In general you want to try and build more chunks of your model as solid pieces and then slice those solids using booleans, rather than trying to go around and build only little separate patch pieces of your model one at a time.

- Michael