Panasonic seems to match the versions of HDWriter with their cameras. I tried to get an upgrade for 2.6 that came with a SDT750. Panasonic refused to give it to me or sell it to me. They said it wouldn't work. The only way I can think of to get a copy is to find someone with the same camera and borrow the disk.
That said, you don't need it nearly as much as when the camera was new. The reason is that editing software has caught up with the AVCHD and 1080p60 formats. My personal favorite is Adobe Premier Elements 11. Since 11 came out, I have rarely used HDWriter. Others like Sony Vegas Home Studio 12. Both are under $100. Both have free 30 day download trials.
Premier Elements has a menu choice to retrieve video clips directly from the camera. I don't know how well it works, because I've not tried it.
If you only want to view your video clips on your computer, you need little. Look for files that end in .m2ts on your SD card. They will be buried a few layers down. Those are the video clips. All the other little files are unnecessary and are only used by the camera and HDWriter. Once you've copied the .m2ts files to your computer, you should be able to watch them by double clicking on them and they will load into whatever default video player you have.
If you had used HDWriter to transfer the files, it would put those .m2ts files in a nice file structure organized by date. You can do that by hand with Windows Explorer.
If your default windows player is balky and unsatisfactory, try downloading the free Splash Lite. If you have a newer Blu-Ray player you should be able to copy the .mt2s files to either a disk, thumb drive, HDD, or SD card. It depends on the Blu-Ray player brand, ports and vintage.
Once you get the .m2ts files off your SD card, you can put it back in the camera and reformat it. Then, somewhere in the menu should be an option to transfer your internal memory to the SD card. You should also be able to see the files if you plug the camcorder into a USB port. The computer "should" see the camcorder as a generic external drive.
Good luck. Remember that Mac owners can't use HDWriter at all and most of them get by fine.