Check if your list is built
From the left navigation, select Contacts -> Lists
Search for the list you just built
If it shows a tag "Building", then list building is in progress. Depending on the number of contacts your requested, it may take some time.
If the list does not show the tag "Building", contacts should have been imported. If not, contact support.
List i built, but not showing in campaign
Any list build in the campaign, gets automatically added to it
If it does not show, try refreshing the page, else follow the next step
Click "Add Contacts"
From the pop-up, select "From your lists"
Select the list and hit "Save"
The contacts will get added to the campaign now