To answer both of your questions, steps 15 to 17 are meant to allow you to install the 13codecx.ax to your wrapper directory. You know how you need to use the "install software" feature to install steam on your wrapper? You need to do the same thing to install bcm1043.exe to the wrapper. Then, search downloads in finder, then search wine in finder from the downloads section. From here you should see a folder named wineskin with your wrapper inside. Then right click on your wrapper and select "show package contents". From there you should find a drive:c folder and then follow the directory I posted to find the 13codecx.ax. The system 32 folder where you paste 13codecx.ax is also in that drive c.
Steps 18 to 19 are merely recommendations to ensure steam will boot up correctly, and also allows you to select other apps besides steam such as wol in a windowed UI. There is a screen options option in the wrapper that allows you to change window settings of your wrapper. I don't understand why, but when you run your wrapper as a windowed desktop, if you don't open any apps right away, the window will close on you which is quite annoying.