WaveLab Elements 9 support aac in and out, in 32 and 64 bits. Prior wavelab uses aac import only, and in 32 bit only. This was through quicktime. Maybe your m4a file is not aac. Do you know its underlying format?
I just tested with the AAC and m4a examples from here: http://techslides.com/sample-files-for-development
and these load fine into Elements 9 (32 and 64 bit) …but strangely these do not load properly, at least via drag and drop, into Elements 8 (they load but have strange looking wave images and are mostly noise).
All of the above files from both sites play without problem using Quicktime and indeed Windows Media Player.
After checking: the file you try to open has the ALAC format (Apple Lossless).
WaveLab should be able to open them, but there is unfortunately a bug…
but there is a solution until a fix: just change the file extension to .caf, and WaveLab will be able to open them, if you use the 32 bit WaveLab version.