Basic/Intermediate/Advanced Discovery Scanner only find bodies, it has some use at low range when run passively but youd'd have a way easier time by assigning it to a firegroup and fire it. Basic DS and Intermediate DS have ranges of 500 & 1000 LS. ADS finds all bodies in the system, you have still to surface-scan them. The passive range might just be large enough to catch the nearby star and nothing else.
The DSS(Detailed Surface Scaner) is an upgrade on your internal Surface Scanner(which is integrated, not a module).
With the DSS (or the integrated SS) you need to select a body, look in its direction and be close enough(depending how big the target is), once the 3 criteria are met, it should start scanning.
Just do it on a star if you havent done it yet. Then try a nearby planet, you may have to get closer for the scan to begin.
You have to wait until it's done and then the scan will count.
And if noone has scanned it before(and sold the data) you'll get your 'discovered by' tag on those surface-scanned bodies when you sell the data plus a bonus for the discovery(And no mater if someone did a scan before you if they havent sold anything yet). Just dont die before you return 'home'.
You need a B/I/ADS to be able to select bodies. But this wont count as discoveries if you dont surface-scan anything.