Welcome to the forum.
With the latest version of Apple's QuickTime Player, you should have a viable H.264 CODEC installed. That you can play the MTS files from WMP indicates that your system has at least one viable H.264 CODEC installed, as WMP only uses CODEC's installed on the system - unlike some other players, such as VLC Player, or MediaPlayer Classic HC, which contain a bunch of other CODEC's, just for their use.
Regarding the "CODEC Pack," which one was installed? Many of those will install a lot more, than just some free CODEC's, and a few of those "other things" have been known to break Adobe products. One, that has caused a lot of grief, is FFDShow, though some users have it, with no problems.
Are you getting a "missing CODEC" error message, when you Import the MTS files into PrE?
Also, with PrE 10, do you have an nVidia video card/chip (GPU) with the latest nVidia driver installed? There is an issue with the latest nVidia driver and PrE 10, but that affects display and playback, and should not yield any CODEC error messages.
Thank you for the welcome and your reply. Regarding QT that is what I thought. I installed the c.net Codec pack which nVidia recommended but I only installed this as a last resort. I do intended to uninstall it.
I do not get an error message until I try to view the images in either the preview pane or the monitor panel. It then says the selected file cannot be played because my system does not have the required compresser/decompresser (Codec) installed. I can see the thumbnails but when I click on them all I get is a blank screen.
Yes, I do have a nVidia card, it is a GeForce GTX 570 v332.21, which is up to date. I suppose I could roll the driver back to an earlier version but as this is the first time I have used PE10 for MTS files I can't be sure how far back I would have to go.
The one thing I have noticed is when starting a new project you have to chose a setting e.g AVCHD 1080 25 but the settings on the camera are 1080/50i. There are no settings in PE10 that match. Is that the problem?
Thank you John for the link. I can't see my card but I will give it ago.
At the bottom right of the screen do you see a message saying "Conforming -name of clip- and a progress bar" If you do that means your clips are still being conformed, you should give it some time. You'll start seeing some of your AVCHD clips play fine one by one.