Position your cursor on the barline in your score where you want the loop to start.
Above the main window you see all the clickable buttons. Look for the one with the vertical line on it.
It's to the left of the "mf" button. Click it. The current barline will disappear. Click the button again.
You will see the Music Bar dialog box. It will display a number of different barlines from which you
can choose. Select the start repeat barline (looks like two vertical lines followed by a colon).
Position your cursor on the barline at the end of the last measure you want to loop.
Repeat the same process as above to open the Music Bar dialog box, but this time select the End Repeat
barline (the one with the colon followed by two vertical lines). You will also be able to indicate the number
of repeats you want in the little box below.
I highly recommend going under trhe Help menu and selecting Help Topics...
There is a lot of useful information there.