I just tested the 6.7 Beta Version.
When i load a Waveform taken with the 6.6.xx, it is displayed,
but i can´t turn on the Filter in the Channels Menus.
If i load the Waveforms with the 6.6.18 oder 6.6.23
i can activate the filter when viewing the Files...
How exactly are you opening files? I suspect that you are double clicking on them. Try opening PicoScope first and say yes to demo. Then use file/open within PicoScope to browse to the file and open it. This should allow you to filter. This has always been the best way to open a data file. This way also fills in the settings if you don't have the scope plugged in.