I assume you mean how to use lock alpha in your own drawings? If you mean where to find lock alpha, it's here:

"Lock Alpha" locks transparent pixels - it won't let you remove or add space, only colour on the space already made.
So, for it to work, you need to make a separate layer for coloring, it also has to be transparent. When you open Oekaki, the first layer there isn't transparent, so either erase everything on it or just get rid of it and make a new layer. All new layers you make will be transparent. First, make an area for your colouring, then tick on lock alpha - now you won't go out of the area you just made!
I suggest you make (at least) three layers; lines, colouring and background. They should be in that order too, lines on the top and background at the bottom.
This tutorial might help with getting the hang of layers;
Step 1Step 2Step 3Step 4Step 5Step 6Step 7Step 8