I believe adding the URL as a custom menu item does not work because the edit screen needs to be in it's own window. I once tried adding it as a web part, adding it to a portal page, and point to that instead. In the end, it has display rendering issues, so I stopped going down that route.
I think a good alternative is to have custom menu item call /Altiris/AssetContractCommon/WebParts/Actions.aspx. This will give 3 links (it's the Asset Common Tasks web part), and the first one will be a link to create items.