If you know the line number, then at the command line you can
dbstop in FILENAME at LINENUMBER
But inside the editor it is easier just to click on the dash that shows up just to the right of the line number and before the code itself. A single click will turn it red, indicating that a breakpoint is there. If there is an anonymous function on the line then you might be asked whether the breakpoint is intended to be in it or on the line itself.
Once the code has started running and you are stopped at a breakpoint, a third way is available: the "Run and Advance" section changes to "Step In" and "Step Out" and "Run to Cursor". You can click on a line and then click on "Run to Cursor" and the program will execute until the line you had clicked on.
Setting a breakpoint is a bit more secure than "Run to Cursor" in that if anything interrupts the "Run to Cursor" (such as a breakpoint) then the editor will forget that temporary breakpoint was set -- but it will not forget breakpoints you set through clicking on the dash.