From your "Home" page on the mobile app:
Click the hamburger iconand choose a Team.
The bottom of the page has the following icons:
Choose the far right icon called SafeKeeper.
On the next page, you will see an Add Content Icon blue folder with a 'plus' sign, just to the right of the "search" box:
Click on the Add Content Icon and add your note. Future releases will include an upload option that will allow you to add content by uploading a file from your phone.