All of my narrations have been WAV files in Camtasia since version 2 so this shouldn't be a problem.
1: Click on Import Media; 2: Browse to your WAV file; highlight it and click Open. 3: This adds it to your media bin. Then simply drag it onto a track in the timeline.
If the WAV file is not being accepted, something is goofy spec-wise or corrupt about that file. Try to import a different file as a test. I still have v8.6 on another PC so I'll grab the screen.
(1) 48kHz sampled WAV files will import, but if there is any overmodulation they will sound very crunchy.
(2) Some WAV files are non-standard in some way, and you have to convert them into a standard WAV file (e.g. via a program like Bigasoft). I noticed this with files created using Garage Band and with some WAV files that Adobe Premiere exported. I do not know what the parameters were which made these files nonstandard -- only that neither Roxio nor Camtasia could play them even though they played within Windows Media Player. After conversion -- which was nondestructive BTW -- both worked fine in Camtasia 8.