I am afraid you will have problems to define micron as length unit in I-DEAS. The important question is : what do you want to do ? Geometry modeling ? Simulation ?
The default global tolerance in I-DEAS is 0.01 mm or 10 microns ! If you wish to create dimensions of micron level you need to set it to 0.0001 mm which is the smallest value allowed (/MO SPE T DF...). And then, you will only be able to define 1 digit on your dimensions !
My feeling (and my experience) is that it is not a good practice since you may hit numerous problems working with a non standard tolerance value. Maybe the latest versions are more robust on that side. Other people may have experience in doing that ???
If possible, I would suggest to use a virtual and scaled model. Replace micron by mm on your dimensions, use mN for force unit. Then, you just have to scale your material properties to make everything consistent...
Hope this helps.