Easy answer: You can't.
I also expected this, since it should be a standard operation if you import data correctly as every band in .pix format has metadata describing the wavelength (working with SPOT and WV data myself). Also the help describes the possibility if you look closely, but it is not implemented.
If you bother support with it:
The spectral plotting tool is being totally re-written in ATCOR 2015 so it will address the issues you are referring to. Geomatica 2015 should be available in a couple of months. The Focus spectral plotting tool in Geomatica 2014 is designed for use with hyperspectral imagery, not ATCOR imagery so it does read all of the required metadata tags.
I tried to validate ATCOR output, but generally its the same for DN data.
Quick and dirty workaround i used if you want to compare it to the spectral library of the USGS for example: Downloading the relevant spectra .asc files one needs and recoding the relevant wavelengths your sensor has to band numbers.