Stuck at step one

I had a corroded battery and need to clean the contacts. This post has detailed steps but I can't get past step one.

The cover just will not come off. I can easily pry up the edges but it feels like there's something in the middle of the top plate that's attached and I don't want to break the mouse.

Step one mentions about moving the pick tool along the edge towards the battery cover and mentions a pin I believe. This just doesn't seem to work and I don't know what to do next.