Nosepass is a difficult Pokémon to catch in Pokémon Brilliant Diamond and Brilliant Pearl. You have to find it by waiting for a random spawn by talking to Dawn or Lucas' younger brother in the town of Sandgem and seeing if they tell you that a swarm of Nosepass is appearing. Once you have captured Nosespass, you can evolve it into Probopass. Here's what you need to know about the evolution of Nosepass in Probopass into Pokémon Brilliant Diamond and Brilliant Pearl.
Similar to Magneton in Magnezone, you will need to take Nosepass to Mt. Coronet and level up once. You do it through battle or you can do it through rare candy. Regardless of how you do it, when Nosepass levels up once in this area, you will be able to receive Probopass and you can now use it in combat and other trainer battles.
The real problem is finding Nosepass, so you may be stuck on this step for some time. You can't catch it underground. You can only catch it after completing the game, so that means beating the Elite Four, meeting all the Pokémon in the Sinnoh PokéDex, and then receiving your national PokéDex. When you reach this point, contact Dawn or Lucas' little sister in Sandgem Town.