The input of a MGRS grid is done in two ways. Based on a known point or present position. Either way, the first step is selecting a GZD using the OSBs, then a selecting the 100,000-square meter identifier by slewing the TDC and finally entering the coordinates with the UFC.
Even though MGRS is the predominant location format used in CAS, I don't blame ED at all for not implementing it yet.