OK. It looks like this in 3.3.2, which is a bit newer version than you are using.
The right hand button in the image is manage files. If you mouse over it in your Moodle (not my image), you will see a tool tip saying "Manage files." In this Moodle.org site, the manage files button is to the *left* of the microphone and video buttons and has a paper clip added to the sheet of paper with a corner folded down, but it still has a tool tip "Manage files."
You click that button, then browse for the file you want, upload it and present a link that students can see.
@Mary Cooch, is there a video that demonstrates how to add a file for students to download to an assignnment module?