We saw Mary Poppins & Bert right in front of the Castle. It was a very enjoyable show. The advice to check at City Hall is the best way to track 'em down.
I have to agree with checking with City Hall. But with my luck of the draw the last 4 times I've gone to Disneyland, Mary, Bert and Mr. Penguin were in front of the castle with the Main Street Band (sorry if that's wrong) and they were dancing with guests to Supercalifragilisticexpialidocious and Step in Time. Good luck...luckily they're an easy bunch to find!
Last year we ran into her just outside Rancho del Zocalo (see picture below). This year we saw her hanging out with the penguins at Refreshment Corner (aka Coke Corner).