I suspect that the code you mean to write was something like the following:

Manipulate[Dynamic, {u, "xxx"}, ControlType -> InputField]

The resulting Manipulate, when exported as a CDF, exports fine.  Howerver the input field will only work if a numerical value is written into it: not if it has a string in it.

To clarify Christopher's comment.  Free CDFs only allow input fields to have numerical values input into them. If one were to allow string values in the input field then within the Manipulate one could use ToExpression to replicate many aspects of the Mathematica FrontEnd.

See the following table for the limitations of the free CDF player.

http://www.wolfram.com/cdf/adopting-cdf/cdf-and-mathematica-comparison.en.html

The code above looks like it might be incomplete. Is that code you posted the correct code?

Additionally, do you see this behavior when previewing the Manipulate as a CDF in Mathematica? To do this, evaluate the statement and go to  File > CDF Preview > CDF Player.
Mathematica should now act as though it were the CDF player.