Never mind, I managed to put a claim in for that address by using the workaround others have mentioned (go to https://www.instawallet.org/ first and then paste the wallet ID in after).  You really should have it set up so that original wallet URLs work to get to the claim pages - the instructional text on the Instawallet front page is currently misleading.

The workaround doesn't appear to work for my other two wallets by the way, they each get a 404-looking error but different to the one I pasted earlier:

The page you were looking for doesn't exist.

You may have mistyped the address or the page may have moved.