As far as I know, that has never worked as long as I’ve been on iCM.
Export the list from IMDb. Copy the movie-urls from the csv. Edit the list on iCM. Go to the tab “By IMDB movie url(s)”. Paste the movie-urls there and save.
It did work at one time, yes. But it hasn’t worked for a while, and since exporting and copy pasting urls from the csv is pretty quick and painless I doubt it’s a priority to fix.
It has never worked since I’m on iCM (which is since early 2012). Just don’t bother with that option. Export the IMDb list. Copy all the movie urls to the tab on iCM that says “by IMDb Movie URL(s)”. Then save.