In general, when you have a question about the inputs for a given screen, click on the context help button (either a button labeled "Help" in various dialog boxes, or the "?" button on the button bar when at the main REM window).  In this case - the duct screen has a wealth of information.

First you need to select what type of duct leakage you are specifying:  Qualitative Duct Leakage or Measured Duct Leakage - so be sure you have the corresponding radio button selected.  Next, depending on what kind of leakage measurement you made, you'll either enter it as "Leakage to Outside" or "Leakage to Anywhere".  The units for Leakage to Outside will also set the units for the Leakage to Anywhere.  The energy load for ducts is based off the Leakage to Outside while Leakage to Anywhere is only used for IECC 2009 and IECC 2012 compliance as well as ENERGY STAR v3.0 Certification.  Duct leakage values are shown on the Air Leakage Report to verify that your inputs are correct and for certification checks.