There are two runs, one for large dogs and one for small dogs. You can reach the park from the parking lot on Webster Street near 36th Street. Mosswood is a large park with the dog park tucked along the edge between the tennis courts and the freeway side of the park. Dogs are not allowed in any part of Mosswood Park except the dog runs and some cops will hold you to this so it is best to enter on Webster.
Mosswood has it's own Yahoo Group for news and information.