After you have trimmend and saved your video, choose the upload menu item:
A popup window opens where you’ll have to login with your YouTube user account:
After sucessfull authentification you have to provide VideoTrimmer access to you YouTube account:
You do so by cklicking the blue “allow” button. After that, your video will be uploaded.
Regarding mobile devices:
The quality of your mobile connection may vary. When it becomes too low, the upload will likely fail. Consider using WiFi connection if available. Uploading via mobile connection can also create additional costs (according to your mobile plane).