Repeat part of a measure using the % type repeat sign.

Here is the end result

Step by step instructions:

1. Enter notes to be repeated;
2. Select the notes (click the first note and press Shift while you click the last note);
3. Press R to repeat the notes (if playback is not important you can select the next rest and adjust its length. Selecting the rest and pressing 5,.,0 will give you a dotted quarter rest in this example);
4. Press V to make them invisible (if you cannot see the invisible notes, use the View menu and check "Show invisible:"
   
5. Select one of the notes near the middle of the invisible notes and press Z to open the master palette;
6. Start to type "repeat" in the search box until you see the repeat sign:
   
7. Double click the repeat sign;
8. Close the master palette by clicking the x in the top right corner (other methods don't work when the search box is active).

The following steps are not always necessary and should never be necessary if you use a single rest rather than multiple notes

9. Press Alt+Shift+E (Ctrl+E in versions prior to 3.0) to put the repeat sign in edit mode;
10. Use arrows or Ctrl+arrows to move the sign.

The following steps are optional if you need more than one repeat sign

11. Select the first invisible note and Shift+click the last invisible note to select them;
12. Press R as many times as needed for the number of repetitions you want:
    
13. Make the invisible notes not seen on screen by removing the check from "Show invisible" in the View menu if you don't want to see them.