As the errors says the partition is already in use thus it would be unsafe to resize it. Ntfsresize has quite a lot sanity checks to prevent data loss. Simply unmount the partition then it will work.

BTW, there isn't any reason not to trust ntfsresize, it's in heavy use for four years by most distros and it's much more reliable than Partition Magic. However some ntfsresize front-end, namely the ones which use Parted (Qtparted, Gparted) may modify the partition table (not the ntfs and your data) in a way very rarely which makes windows unbootable. This can be easily fixed by setting disk access in the BIOS to LBA. On the other hand, Partition Magic is able to mess up real data in a really unfxcable way. And the partition table too.