Two possibilities.
1) is this enabled?
Tools > Options > File Types > Data > Disk Image Files (iso, img, nrg)
2) The files may already be imported, but the view is not showing of ISO files. Verify by creating a new Smartlist, click the Import/Export button, and enter exactly:
~d=a
Then, search the list for your ISO files. You can do this easily in the Search area, in the upper right, by entering
[File Type]=[iso]