Choose msContents from the list. Put the cursor after the start tag for msContents and repeat the Insert Element command. As long as your document is valid, you can also use the Tag Region command on this menu to tag a previously selected stretch of text. If you get irretrievably confused, try C-g to abandon the current editing commands. When you need to insert an attribute value, you can type it in directly, inside the tag, or you can let emacs put it in for you.
Press return to add it to the document.
Edit attributes edit the attributes of the current element Most of these have keyboard shortcuts, which are given on the menu and also in your crib sheet. This is the part which checks that your document is both well-formed and valid, and complains if it is not. Most such systems will try their best to help you when they have found errors, but you do need to understand what is underlying the messages. Your version of emacs uses an external validator called nsgmls to check the syntax of the fuiles you create. The command line used to validate the currently open file will appear in the minibuffer at the bottom of the screen.