On line 26 of wurfl_admin_inc, the code checks for the existence of a file or directory at $library/cache. However, depending on what is in wurfl-config.xml this may not be where wurfl needs a cache to be. I'm afraid I don't know how to get the WURFL API to tell me where it is expecting the cache to be according to the config file... this would presumably be quite a quick fix for someone who knows how to do that.