Cache is located around the back of the building, to the right of the entrance if you are standing in front of the building, facing it. If you visit the museum you will end in the cafe which puts you close to the cache. The cache is located in the garden beside the cafe. Look at the base of the silver shed.
*note* It sounds like local folks enjoy assisting you in finding the cache. However, if someone asks you for money if they assist you in finding the cache, this was obviously not part of the original intention.
Small note on cache maintenance:
There is a local cache guardian who will assist.
Name and contact info on file with geocaching.com.