In iTunes Library -> Music view, select the left of the three sort options towards the upper right so it lists individual tracks (rather than album artwork).
Now select all of the tracks from both CDs of the album you imported then right-click and choose 'Get Info'. You can then update common info for all of the tracks such as Artist and Album. You can also input a common figure for the total number of discs such as "(blank) of 2" if it is a 2CD collection. It should then be trivial to select all those tracks on CD1 and similarly highlight them all and make them disc "1 of 2", and those from the second CD making them disc "2 of 2".
Provided the other meta-information is correct (you can easily edit other fields in iTunes if needed), then they should then be listed correctly.