Make sure you have returned to your course page by clicking on the short name in the navigation block. When creating a course I usually remove all of the blocks except for the Navigation and Settings blocks. This forces you to justify to yourself on which blocks to add from a teaching/learning point of view.
To add this new block we need to scroll down so we can see the Add a block option:
Click on the drop down and select the Random glossary entry from the list (it is near the bottom of the list).
Once it is added you will need to configure it using the edit icon, so click on the edit icon in the new block immediately above the Add a block menu box.
Some of the settings are already filled in, and in fact you don't need to change many settings. However, to start, change the Title to Important terms. This is the name that the block has in the course view page. Often people choose to have no block name, but this may not work with your theme...