All right, good and good on the first two.
I'm not surprised that you get those weird icons, as I doubt you have renpy editing software on your computer. They're .rpy files, and they need to be dropped into the Steventon Street game folder. You should be able to do that without extracting.
Are you able to open the file explorer where SteventonStreet.exe is located (for me, that's a folder labeled SteventonStreet-1.8-pc), then (in a different window) open the zip folder without extracting, and drag and drop the two DLC files into the folder labeled "game"? (I'm sorry if these instructions are unclear. If I've explained poorly, I can record my screen and demonstrate, then send the video file to you.)