Hey...I haven't actually done this tutorial before..but they way I see it...there are two ways (using the split polygon tool)...to get the results as the tutorial shows....
1) add more splits(and create more edges) to the face so that you can get the desired shape
or
2) under the split poly tool options increase the subdivisions...this will create more vertices on the newly created edge...and to move thos new vertices into place so that it looks like the tutorials'

hope that helped..good luck
TheAnswerRP03