in Cadence 6.15 /ViVa you've to place two markers ("m") before you can use the delta marker
==> Place marker 1, place marker 2, mark both (usign shift key) and press shift + "d"
Other possibility: use the 'new' so called "AB marker". It has the functionality of the old delta marker of Cadence 5.
==> Press "a" to set the 1st marker, press "b" to set the 2nd marker. Additionally, the delta in x and the delta in y direction as well as the slope will be shown.