Open mybeam.scad, scroll to the bottom where you see "beam(15);" Change the number in parentheses to the desired number of holes. Choose Design→Compile and Render, then choose Design→Export as STL; Once that's done, you can print the STL.

Some other items of interest:

* angle_lever.scad: It's like mybeam.scad, except you can create objects like the 4x4 technic-compatible lever.stl attached. Change "angle_lever(4, 4, 53.5);" to whatever you want.
* bush.scad: Use this to create bushes and half bushes. Change "bush(beam_width, 3, 3);" to whatever you want. When you print it, you'll need to break off the little tabs ("mouse ears") that hold it in place.
* gear.scad. Use this to create gears. Change myGear(number_of_teeth=40, holes_row=4, holes_col=2); to whatever you want. Requires gears.scad from MCAD (https://github.com/D1plo1d/MCAD)