Serial numbers are matched to the particular product -- if you had a suite or upgrade product, rather than the standalone application, you would need to download the same installer as the original (to match the serial number).
After reading your response I went looking for and found the original purchase download. Enter the serial# and works. Now asks for the qualifying sn# and I have a Creative Suite2 which includes InDesign. That serial# doesn't work. But worked before?? I have had InDesign on my iMac for 2 years without a glitch until now. More curious as to where 'bouts of the Roman.txt file.
The only place I see the roman.txt file on my Windows system is in the type support files for Acrobat 9. No association with CS5.5 at all.
We really can't help with licensing and serial number problems beyond the basics of where to look for the numbers. You need to Contact Customer Care