To use the Microsoft Configuration Manager (web.config) as your configuration mechanism you must include the Owin.Framework.ConfigurationManager NuGet package into your solution.
Note that this configuration option is very limiting because the web.config file does not allow you to store structured data without defining configuration section classes. This design makes it impossible to fully implementat the Owin Framework configuration design, and you are limited to just storing simple name/value pairs.
If you need backward compatiblilty with an existing web.config based configuration mechanism (for example because your continuous delivery system knows how to make changes during deployment) then this is a useful short term stop gap, but long term the even Microsoft have admitted that web.config design is weak and moved away from this in their latest software.