I don't have time to check this tonight, but don't forget that you can always look through the Sandbox Log.txt file (in the (My) Documents\Universe Sandbox\Logs folder) and see if there are any errors listed there.
While this probably isn't the problem, Universe Sandbox doesn't deal with more than 16 digits of precision. So any number that's longer than 16 digits will be rounded to 16 digits (the decimal place can be anywhere however).
So...
3141592.65358979323846264338327950288419
Will become this...
3141592.65358979
And this...
.00000000000012345678901234567890123456789
Will become this...
.0000000000001234567890123456
Oh...
<Object> must be something in the Universe Sandbox database (like Earth or Mars or Ceres).
Then use <Name> to change the label.
<Object>Pi 1</Object>
<Mass>15.7079633 earths</Mass>
Should become something like this...
<Object>Neptune</Object>
<Name>Pi 1</Name>
<Mass>15.7079633 earths</Mass>