Dex Nav isn't the same as PokeRadar. For reference, Dex Nav exists in ORAS, PokeRadar in XY.
With the Dex Nav, the basic run down is:
- open dex nav, tap the Pokemon you want, then press "search"
- when the Pokemon appears, fight it, then either faint or catch it; running away breaks the chain
- when the battle's over, walk out of grass again and search for the Pokemon as soon as possible (do not run or bike; they'll scare away any potential dex nav encounters and biking breaks your chain)
- if the Pokemon you want doesn't appear (Dex Nav will say something like no signal), walk around until another Dex Nav Pokemon appears, faint it, then search for your Pokemon again
Here's a reddit thread detailing it.