Create the alias by clicking on Domain Pointers, and then when you create the alias, be sure the Create as an Alias box is checked before you click on Add.
Then it will do what you want. If you do not have the box checked, it'll do a redirect.
If this does not appear to work:
- delete the DNS records of the domain you wish to park in admin level > dns administration
- log back in as the user of the domain
- recreate the alias in advanced tools > domain pointers