Are you using Mechanica in integrated mode or not ?
if integrated mode, just create a new CSYS from within mechanica as you use to create one within Pro/E, BTW, CSYS might be created on the fly when creating your BCs. Current CSYS turns into green...
In standalone mode 3 points might be created as soon 1-2 vector is orthogonal to 1-3 : simple as that
Cheers
Ph.V