Enabling Intellisense for Custom Sections in .config Files

As the other answers say, you need to provide an XML Schema document for your custom configuration section. There’s no need to add the .xsd schema file to some global directory; you can reference it directly from your custom section in the App.config file:

<configuration>

  <!-- make the custom section known to .NET's configuration manager -->
  <configSections>
    <section name="customSection" type="..." />
  </configSections>

  <!-- your custom section -->
  <customSection xmlns="http://tempuri.org/customSection.xsd"
                 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                 xsi:noNamespaceSchemaLocation="customSection.xsd">
    ...
  </customSection>

<configuration>

The xmlns attribute is merely there to set a default namespace, so that you don’t need to set it on your customSection element and all of its child elements. (However, do not place the xmlns attribute on the <configuration> element!)

The customSection.xsd contains the schema that will be used by IntelliSense, for example:

<xs:schema id="customSectionSchema"
           targetNamespace="http://tempuri.org/customSection.xsd"
           elementFormDefault="qualified"
           xmlns="http://tempuri.org/customSection.xsd"
           xmlns:mstns="http://tempuri.org/customSection.xsd"
           xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="customSection">
    ...
  </xs:element>
</xs:schema>