If you are creating new users via the GUI you can click on the Destination button in the ribbon and then select the OU where you would like the users created (see below).
Alternatively, you can use a column named destinationOU and use the distinguishedName of the OU. Below is an example:
The destinationOU column can also be used to move existing users to a new OU, below is an example, note the Modify column with a value of True tells the program we are updating existing users.
There is a setting under Options which prevents existing users being moved even if the destinationOU column is present. Check the status of this setting if you are having trouble moving users.