i did buy the software today, so that i can read and write to my second partition. its a windows7 with NTFS.
but now i can't see on the sx in the system preferences my bootcamp windows7 device ?
when i deactivate tuxera and reboot, i can see it again.

i know some workarounds (alt-key,boot champ,..), but this should be not the solution !

why i can't choose my ntfs partition as boot device ?

i know a other software where its possible and is cheaper. i think it was not a good choice to buy it today

Not sure if this helps, but the FAQ said this:

Q: Why does all NTFS drives disappear from the "Startup Disk" preference pane after installing Tuxera NTFS for Mac / NTFS-3G?

A: Apple does not allow any volumes that are mounted with third party file system drivers to appear in the "Startup Disk" preference pane. This is hard coded, and something that we cannot change.
However, there are numerous alternative methods for booting into Windows:
Hold down the Option key during boot (or Alt for non-Apple keyboards).
This brings up a boot menu where you can choose which volume to start your computer from.
Install the rEFIt boot manager for better control of the boot process.
Disable Tuxera NTFS for Mac / NTFS-3G (using the preference pane), unmount your NTFS volume with Disk Utility and mount it again.
Your volume will now be mounted with Apple's read-only NTFS driver, and the "Startup Disk" preference pane will behave as usual. Make sure to reenable Tuxera NTFS for Mac / NTFS-3G afterwards.
Use the command line utility bless to set the disk that should be used as default startup disk (see man bless for more information).