The F2 key acts as a toggle. It will set a breakpoint on a line that has no existing break point. Alternatively, it will remove a breakpoint if one already exists on the highlighted line. This only works if the line is an executable statement. So pressing F2 when the cursor is over a comment line will have no effect. It will also have no effect when the cursor is over a declaration. Breakpoints are marked by a white bar. Once you have set the breakpoints required you can continue the program by pressing F6. A message box appears if the line is can not be used as a breakpoint.
You should take care when using breakpoints. If the line of code is never executed, due to an IF condition, the program will not stop.