Unfortunately, you cannot dimension directly to a pad edge. This is a request on Mentor Ideas (Idea D8038) that I encourage you to vote on.
As a workaround, you can create a rectangle the same size as the pads on a User Layer and dimension from edge-to-edge on those instead. This should let you put in all the dimensions you need. You may find RMB > Snap > Toggle Hover Snap useful in creating the rectangles the same size as the pad. In the picture below, I created some pink rectangles on a User Layer and was able to dimension between them.
Just saw Vern's comment - if you just need measurements (not dimensions) that will certainly work. Just make sure to switch into 'Edge Mode' rather than 'Center Mode' for the measurement.