To have a chance to catch Suicune, you must first meet it in three different places in Johto. Right outside of National Park on Route 36, the middle entrance of Mt. Mortar, and next to the Poke Seer's house in Cianwood.
After saving the director in the Radio Tower, he'll give you a Clear Bell. Go fight the Wise Trio inside the house that leads to the Tin Tower. They each have a Noctowl, and one of Eevee's old three evolutions. After beating them, go to the Tin Tower, where you'll fight Suicune. Hopefully, you'll catch the level 40.