Only the processes belonging to green cursors can step into functions. Remember that while your program is stopped at a breakpoint or watchpoint you can split or merge green and amber cursors.
To run the program and execute the next statement (Step Into)
While the program is paused in break mode, click Step Into from the Debug menu. The debugger executes the next statement in all processes belonging to green cursors, then pauses execution in break mode. Note that in general there may be more than one green cursor and processors belonging to different cursors may have different "next statements". If the next statement is a function call, the debugger steps into that function, then pauses execution at the beginning of the function.
Repeat step 1 to continue executing the program one statement at a time.
If you step into a nested function call, the debugger steps into the most deeply nested function. For example, on the line of code fun1(fun2()) the debugger steps into the function fun2, then pauses.