You can't choose what Pokemon appears but you use it by:
You walk into the grass that shakes or shines. If its not the pokemon you want then run, and try it again. Walk another 50 steps and push Y again. Then try again, and once you find the pokemon that you want to chain, kill it OR capture it. Then right after the battle the grass will shake or shine again.
You can find Smeargle on Route 208 and 212 North using the Poke Radar.