If you can't see this button, first scroll down to the bottom of your page and click the Discard draft button. This will clear any previous drafts you were working on.
If you still can't see the button, check that there are templates set up on your school's site from School > Communications > Templates.
If there aren't any templates set up in the Email, SMS or Letter tabs, you won't see the Load Template button (you can't use the System templates in mail merge communications).
If you need to set up templates, you can see how to do this here: Using Email/SMS/Letter Templates