Try clicking the refresh link on the bottom right of Install from Web. The JSONP call has been failing randomly lately and we're already working to improve this. If that doesn't work you'll have to download manually from our downloads page and install manually.
www.joomlapolis.com/downloads
Note for Install from Web to work you must be logged into Joomlapolis for it to detect your login state. Do not use private browsing modes (e.g. incognito) as they will block the cookie access.