To get to the 520 trail from the Lake Sammamish you need cross the bridge in Redmond (as if you were going to Marymoor), but instead of continuing under the bridge again get out onto the sidewalk. To the left is Redmond to the right is the on ramp to 520 - go to the crosswalk and cross straight across - you'll see the entrance to the trail right in front of you protected from the 520 on ramp by a wall.