Okay, I've re-tested this on Nitro 10 & 11, and the directions below should let you do this without issue.
Depending on if you have a text form field, or a more general box with text, there's two separate ways (it's the same for Nitro 10 & 11).
For text form fields:
Under Forms, click Select Fields, and select your text field. Under the General section on the left, there's a Rotate option. This allows for 90 degree rotations left or right, and resetting back to 0 degrees.
The other way is to use the Edit button- if you see a blue box around the text when you click on the Edit tool, that's this kind, and you should see a four headed white arrow when you move your mouse over the edges of the text box. Click once when you see this, and hopefully you'll see a blue arrow. This arrow will let you free rotate the box.
If you don't see this blue arrow, it's probably because your item is an annotation- using the Type Text tool, and then wanting to rotate the text is the most likely reason here.
To be able to rotate the text box, select the text, and right click on it. Choose flatten selection. You can now repeat the steps above with the Edit button.