difficulty pasting in muse score

Make sure to have notes or rests of the proper duration in place before pasting

I'm not sure exactly how you are trying to do this, but music notation itself is the "measuring ruler". If you want to paste something onto beat three of a measure, for example, just click the note or rest current on beat three, and your paste will happen exactly there. If the measure currently doesn't have anything on beat three - like there is a whole note there now, or the measure is empty - then of course you need modify the notation so there is something there - music notation don't allow something to just magically appear on beat three without notes or rests adding up to beats first. So, assuming it's an empty measure, enter a half rest first, then you can paste onto beat three.

If that's not what you mean, please attach your score and describe in more detail what you are trying to do so we can understand and assist better.