[Haskell-beginners] Stack doesn't find downloaded module

Hi! I downloaded the following package https://www.stackage.org/lts-13.29/package/multiset-0.3.4.1 using the following cli (Linux) command: stack install multiset. It was supposedly successfully installed but when I try to import it stack says it couldn't find it ('Could not find module ‘Data.MultiSet’). I've tried reading haskellstack.org's documentation but I've not been able to find a solution. Well, thanks in advance for your help Mike