Entsel basically returns a list containing the following:
Code:
(<EntityName> '(X Y Z))
The '(X Y Z) is a list representing the XYZ coordinates as a 3d point of where the cursor was when the user clicked the mouse button. So in the case of fillet, it "should" work if that point is anywhere on the entity. So first you'll have to calculate a point on the entity (could be an endpoint, but with fillet I don't think so). You may want to look into the vlax-curve-XXX functions ... they help a lot when using arcs & polylines.
To create the list after you've obtained the point you simply use the list function:
Code:
(command "._FILLET" "_Radius" rad_val
(list ent1 pt1)
(list ent2 pt2)
)