You can set unit system to either Imperial and Metric. To change the unit system go to File > Options menu and open application options dialog.
Length, mass, time, temperature can be chosen within the unit system. Force unit can be chosen independent of length, mass, time units.
It is important to note that, any Force / Area (such as stress) input and output will be in Force / (Length2) units, and any Force / Volume (such as unit weight) input and output will be in Force / (Length3) units.
Before sending model to OpenSees for analysis, Force, Force / Area, Force / Volume quantities are converted to units that are consistent with base units (Length, Mass, Time).
Rotational displacements at nodes are always displayed in radians.