If I type +5' (plus five feet) into the two X-fields, the original dimension is shown with nothing changed.
You need SH3D 6.0 (latest version) for calculation to work in the X fields. How, or rather if, this would work with foot/inch/fraction, I do not know, but it seems to work with Inch/decimals. So if you have 6.0, try changing to Inch/decimals in Preferences. Or, of course, you could take a leap in to metric... 
ok