Yes, this can be done on the page with the download link.
1) create a new form with a text field for the password and a submit button. make sure the method is set to post, and the actin to go to the same page.
2) open the server behaviors panel, click the plus button and go to:
WebAssist -> Cookies -> Set Session Value.
for the trigger, click the lightning bolt and select the password form element, do the same for the value. for the name give it a friendly name like:
3) Open the Security Assist access rules manager from the top Dreamweaver menu bar:
WebAssist -> Security Assist -> Manage site access -> Access rules manager
and create a new rule as:
Name: Logged On
Value: click the lightning bolt and select the userPassword session binding
Criteria: select Equals (=)
Compare to: Enter the password the users should use
4) Now select the download link on the page and use the Security Assist show region server behavior. open the server behaicors panel, click the [plus button and select:
WebAssist -> Security Assist -> Show Region.
Select the Logged On access rule.
this should cause the link to not show when the page first loads, and then show if they use the password form and enter the correct password.