Adding Isometric dimensions in Revit is a relatively simple exercise, the conventional ‘Aligned’ dimensioning tool works fine, the trick is too correctly orientate the Work Plane the dimensions are placed on.
By default, the ‘Work plane’ will be positioned parallel to a level, the ‘Show Work Plane’ tool will highlight its current location with a grid.
Use the ‘Set Work Plane’ tool to relocate the Work Plane. Several options will be offered, the easiest choice is ‘Pick a Plane’, simply move your cursor over the appropriate face (or edge) and select it.
Any dimension placed from that point onward will be placed on the current ‘Work Plane’. The image above shows the ‘Work Plane’ on the side of the desk, therefore that’s where the dimension is positioned.
In Summary, use the ‘Set Work Plane’ tool to define the location of the Work Plane, then place the desired dimension.