The MEMORY option only works with non standard cables. If you are testing a standard ribbon cable the tester will automatically stop at the end of the cable and the PASS LED will be on.
If you want to test a non-standard cable then plug it in and press the TEST button. The test will naturally fail as the cable will not be reconised as a standard cable. Press and hold the test button until the MEMORY LED comes on. After this the cable will be automatically tested against whats held in MEMORY.