I had the exact same problem. I sent them an e-mail, and this was what they told me to do (p.s make sure you select the radio button corresponding with your voucher type. I think mine was the last type):
1) Enter your first and last name into the spaces provided at the top of
the passholder entry page
2) Using the third pass mask, type your date. Using the drop down menu,
select the the letter code on the back of the exchange voucher.
3) Enter the number after the code from #2. Ignore the TR information and in the last area, type 049. (personal note - I'm sure this was what was causing my problem - I had a tracking number that was 2 digits, a space, and then 2 more digits. Once I typed in 049, it worked perfectly.)
You may also want to check the following:
Please check your PRIVACY SETTINGS to ensure they are set at Medium.
These settings can be found by following the steps listed below.
1. Select Tools from the top menu area
2. From the drop down menu, select Internet Options
3. From the window that opens, select the tab for Privacy
4. In the top part of this tab you will find your Privacy setting. This
can be set from Block All Cookies to Accept All Cookies. The minimum
setting to allow you to log in would be Medium.
Clear your Temporary Internet Files CACHE and Cookies:
1. Select Tools from the top menu area
2. From the drop down menu, select Internet Options
3. There should be a section marked "Temporary Internet Files" on the
tab marked General. In this section there should be at least one button
that says "Delete Files". Select this "Delete Files" button. Another
box should pop up asking if you want to delete offline files also.
Check yes and then select OK. You should also see abutton to "Delete
Cookies." Please also click this box. The system will then go about
deleting the temporary internet files from the computer. Don't worry,
this does not affect favorites or any bookmarked items, but will clear
the cache and cookies.
If you have a firewall sometimes it will block our website. If your
firewall gives you an option to accept the site then simply click yes.
However, some firewalls will not give you that option. If that is the
case then you have to disable the firewall long enough for it to allow
our site to load.
Finally, we suggest when you try logging into the Passholder section of
the web site, make sure your are logged out of your disneyworld.com user
ID/Member name. Sometimes this can prevent you from being able to login.