No enrollement button on iTunes U to enroll to the course. Please advice.

You are providing no context within which we can understand your problem. Please provide more thorough steps, preferably with screenshots.

I have the same issue, and responded with this screenshot on a different thread. For the courses shown, I enrolled on a different device with the same Apple ID. But I can't enroll in anything with my iPhone4S because there is no + sign next to Catalog.

The enroll button has apparently been moved in the iPhone version:

• Tap Catalog in upper left
• Tap Featured in lower right
• Scroll to the bottom
• Tap Enroll
• Enter the enrollment code

I guess Apple decided they could make enrollment less obvious on the iPhone because most invitations are emailed. /shrug

Does that solve your issue?

Yes, thank you!