This message you see is a warning or information to remind you what IIS7.5 does when you change/add mapping on the application level, as in that case it will update your web.config file with the new/changed mappings. This would mean that if you added the mappings on application level and later copied the web.config from your Visual Studio's project folder, it would overwrite/invalidate the mappings you just added.
I usually find it more convenient to add my mappings on the website level, as then they are there for applications I decide to create later.
When you add a new mapping, you will be asked the question if you want to allow this ISAPI extension, in which case you must respond Yes.
Looking at your original "StaticMapping" error message, it indicates that you may have the mapping active for static files only. Pay attention to the Request Restrictions button when you are adding the mappings and make sure that "Invoke handler only if request is mapped to" checkbox is cleared (unchecked).
Hope this helps,