Hi Victor,
At present, annotations can be created only on the "native" orientation of the image (i.e., if axial slices were acquired, you can annotate only on the axial view of the images), which is what the XNAT-OHIF viewer shows you when you first load up the image. At present, you can't create annotations on the orthogonal views shown in the 3D MPR mode (although you can view previously created contours and masks in MPR view).
Best wishes,
Simon