Just repeated the sequence - same problem. About to reboot and try again from a clean start…
Rebooting didn’t help - same problem.
For info: DS 188.8.131.52 Pro (64 bit version) - checked the change log and it doesn’t appear to be a known bug that’s fixed in a later release. Can’t find a list of known unfixed bugs anywhere.
Edit (29 Sep ‘12): Still got this problem. However, since the ‘path’ for a parameter can be edited (by double-clicking on the parameter name as Rob explained) that allows me to get round the issue. Not a solution, but a workaround.
I’ve started a new thread Simple Material Mixing With DS4.5 Shader Mixer - Advice Please? thread over in the DAZ Studio Discussion forum focussed on my first Shader Mixer project.
Big thanks to Pen, Richard and Rob for the help getting starting with this.