If you use the "standard" rotate tool... you can't set the center point of rotation on a group of points.
There are some "custom" tools that users of Moho have created available in the scripting forum that allow for setting the center of the rotation for a group of points.
However... the center point is not "settable" by a numeric value... at least I don't think so.
Plus... rotating points is actually... just a sophisticated translation of points.
If you rotate a layer, Moho is keeping track of ROTATION.
If you rotate points on a vector layer... all you are doing is TRANSLATING points to a new location that APPEARS to be rotated.
If you key framed point rotation as an animation... the points will not "spin" or "rotate" they will move in a straight line to the new location.
Sometime this will look like rotation.. but it isn't really.