In WF5, and prior, you cannot change the spring rate unit within MDO, you get these units as a function of the unit system your assembly is in. All derived units are based on these fundamental units used by your selection. A value with units of lbm/sec^2 is a force/length unit. And 1.0 lbm/sec^2 = 2.59E-3 lbf/in. If you must use the Imperial system, you'll need to change your assembly's unit system to a force-based set, like "IPS" (in, lbf, s) and you'll have lbf/in for all springs. Or, in metric, use the "mmNs" set and you'll have springs in N/mm for a rate.
In Creo Parametric 2, you can change the spring unit in the dashboard, independent from the assembly unit system. I think this was done in Creo 1 as well, but I don't recall.
Hope this helps,