The current new order details page is faulty in so many ways. One of the many problems is that the Resolve Problem button does nothing.

You should however be able to bring up the purchase or sale in a list of purchases on your My eBay sold items page.

Scroll to the purchase in question, left click on the More Actions link and then go down and left click on the Resolve a problem link.

Or you can also go directly to the Resolution Centre

http://res.ebay.com.au/ws/eBayISAPI.dll?ResolutionCenter

then select "The buyer and I agreed to cancel a transaction" and click continue then enter the relevent item number.