Thanks for helping to bring this to our attention. You should be able to adjust the quantity you have available in inventory by going to your "Manage Inventory Items" page and selecting "Adjust Inventory" from the "Actions" drop down menu next to the desired one.
I just tested this out and I am also seeing the page go grey and get stuck when I select this option using Internet Explorer, but it seems to work with Firefox. It appears there is a problem with this feature so I'm going to bring this to the attention of our engineering team and they should be able to get it fixed with a future site update.
For future reference, the quickest way to get a response from an Auctiva representative is to file a support request using the form on the following page of our site: http://www.auctiva.com/help/requesthelp.aspx