to use dexnav for searching for a specific Pokemon, you need to have already caught one. Catch one Heracross, then select Heracross on the dexnav. Every time it works using this method, it WILL be Heracross. make sure to actually encounter Heracross, and after enough encounters, it will have moxie (dexnav will show this after your encounter count is high enough).
Hope this makes enough sense for you and helps