in Inventor 9 use dimension the arc (radius or diameter) & right click. You will see a drop down menu of choices including: radius, diameter, angle, arc length. Just pic arc length & you'll be all set.
No arrow next to diameter. When I select diameter, the dim changes to a diameter dimension and the right click options change to substitute radius for diameter. No other changes noticable.
In an IDW you can dimension it. In an IPT (which you are in due to the options you described from a right click) you can use the formula for arc length (Angle*rad*pi/180) as a custom Parameter if you dimension the radius and the angle of your arc. Otherwise there is not an arc length dimension.