By default BC Log expects Bone and Muscle Mass to be entered as a weight value. If you prefer to track these as a percentage you can set this easily.
Navigate to "Settings / Metrics to Track". From here you can select the "Bone Mass" or "Muscle Mass" metrics. You should now be able to select "Track as Percentage" as desired.
When you change from Weight to Percentage tracking, or vise versa,, BC Log will attempt to update all the values you have previously entered. If you have not entered a Weight for a given date though it will not be possible to calculate a percentage for that date and so the value will be left blank.