Edit Text

To Add text to a dimension, click the object you wish to dimension, ie the line, arc, circle, but before you click to place the actual dimension text, type "m" into the command line.  This will bring up the Mtext window where you can add text to the current dimension text.  The cursor is by default in front of the existing dimension value, so you will need to hit the right arrow a couple times to get the cursor behind the dimension text, and then you can type in whatever you want to add to the text.