For those of us that started out using Autocad and then switched to Revit, you may be having a bit of a time finding the Fillet command. The command does exist in Revit, but how you get to it is a bit unexpected.
Let's say you have two walls or lines that you want to apply a fillet to with a radius of 5'...common sense may tell us that it should be on the modify tab, but sadly we would be mistaken. To access the fillet arc command (as it is called in Revit) you need to start either the wall or line tool to gain access to this feature.
When you are using the fillet arc tool on a wall, make sure that you are using the same element type as you are going to be applying the fillet to. The easiest way to insure this is to select one of the walls (or lines), right click, Create Similar. This will bring up the Wall Context Tab.
On the Wall Context Tab, set your preferences on the options bar (justification doesn't matter since it will center the arc wall on the two connecting walls).
You can specify a radius beforehand by placing a check mark in the radius box and then specifying the radius value...otherwise pick one wall then the other and drag to create the radius.
If you need to modify the radius you can do so by selecting the temporary dimension and specifying a new value.