When you open the debugger tab, you should see the source for the main
entry-point for your app loaded in the debugger.

In order to browse around more of your application sources, click Libraries
(top right) or use the hot key command ⌘ + P / ctrl + P. This will open the
libraries window and allow you to search for other source files.

Setting breakpoints

To set a breakpoint, click the left margin (the line number ruler)
in the source area. Clicking once sets a breakpoint, which should
also show up in the Breakpoints area on the left. Clicking
again removes the breakpoint.

The call stack and variable areas