You'll need to edit this is Acrobat Pro. Text field alignment cannot be set in InDesign CS6.
Normally, text field alignment is flush left. However, you can open the PDF in Acrobat Pro. Choose Tools > Forms > Edit Forms. Double-click the text field which is having the problem. On the Options tab you can set alignment.
thanks, i've checked "multi-line" and choose "left" it seems ok now!