The straightforward way is to import the course into the Explore web site, then sync it to the 66i. When you start an import from the map page, you have an option to import courses.
There are other ways. You can export the course to GPX, then import the GPX to the Explore site and sync. Or you can drop the GPX directly onto the 66i.
I don't know if it's possible from the mobile app. I don't even see a way to export an activity, and courses don't seem to be exposed at all. OTOH, I'm not very familiar with the mobile app.
From the web site, select the course so that the detail pane is displayed. In the lower right corner of the detail pane, click the "..." (more) button. Click the "Download (GPX)" option.
If all you have is an activity, you can also export that. Display the activity, click the gear icon in the upper right corner, click Export to GPX.
