MODFLOW will save the concentrations in a file with the extension .conc. You can import the concentrations in the same way you import heads by selecting the .conc file when you import model results.
Select "File|Import|Model Results" and select the .conc file and import the data for any time steps that you would like to display. In the illustration below, the contours used log transformed values.
Screen capture of ModelMuse showing the grid both colored and contoured with the final concentrations. The lower limit for both was set to 0.01.
