Step 1: Aquire a scouter with the dragon radar feature. This will be listed as a 'stat' on the scouter similar to stats on other items (such as +1 Str, etc.), however the stats on scouters are not translated yet. I believe all scouters that are 'superior' quality or better have the ability to detect dragon balls, and I know you can find one at the scouter merchant in Korin's Village for 45k.
Step 2: Wait for the dragon ball hunt (a dragon ball icon will appear in the bottom right corner). If you play on the Taiwan servers, you can find the hunt times
here. Once the hunt is on dragon ball MOBs will be spawned in, randomly replacing some of the already spawned MOBs. After this happens, every MOB killed has a (unknown?) probability of respawning as a dragon ball MOB.
Step 3: Hunt and kill dragon ball MOBs. Make sure your scouter is turned on, by pressing 'T' so the little scouter icon next to your character portrait is lit up. Next, by looking at your mini-map you will be able to see all dragon ball MOBs in your immediate area. They will appear as a little 1-star dragon ball icon in the mini-map. If you don't see any on your mini-map, then you need to either keep looking, or kill some MOBs until they spawn in.