Did you create an account in Samsung Apps, and then enter your login and ID in the Settings menu within the App? If so, press the red "A" button and log into your Samsung Apps account. Then you'll be ready to view your YouTube account (and other associated accounts).
As far as missing videos, YouTube manages the content that is available. In some cases, users have restricted videos or limited viewing capabilities, and those limits may extend to the App. Since YouTube manages it, they would have to address that.
I recently purchased a Samsung TV (Believe it's this one, or there abouts, UN46C6900 ) and for the life of me can't figure out how to tie my youtube account to the youtube app.
Also, videos are definitely missing from the online version. Am I doing something wrong?
Any help is much appreciated!