How do I add a file to a zipped file when using 7-zip?

I used to be able to drag & drop and add a file to a zipped folder. Now I cant when using 7-zip. I upgraded to a new computer with windows 8 (nightmare) that has 7-zip and I would either like to be able to add files to a zipped folder or install the old version of zipping files from Windows 7.

The "add" button inside the zipped folder on 7-zip does not work. It seems once I select the files I want to zip I am not allowed to add additional files to the zipped folder. I used to be able to do this using windows 7.

I don't understand a lot of computer lingo like "archive" "extract" "directory" (I really try to but seriously, I just get more confused when I try to look this stuff up) so please simplify any answers as best as you can. I apologize if my answer is already listed somewhere, I did look but only got more frustrated and confused.

All you need to do is drag the file you want to add into the 7-zip file manager window (the one you get if you right-click the zip folder and choose 7-Zip->Open Archive)