How do I tell ReSharper that an attribute means that a method is used?

You need to use JetBrains Annotations, and mark the attribute with an MeansImplicitUseAttribute. You can either reference JetBrains.Annotations.dll directly, or you can copy the annotations source code (from ReSharper / Options / Code Inspection / Code Annotations) into your solution.

If you need to annotate some external assembly you don't own, you need to create an External Annotation file (xml) in the following folder: %ReSharperInstallDir%\Bin\ExternalAnnotations. There are plenty of examples, you can just copy some.

The external annotations file can also be in the same path as the DLL if you name it DllNameWithoutExtension.ExternalAnnotations.xml.

There are plenty of examples, but I wanted to be a little more explicit in case you don't want to track down an example. :)

Create a file with the name of the attribute's assembly (.xml) in %ReSharperInstallDir%\Bin\ExternalAnnotations. For example, I made Microsoft.VisualStudio.QualityTools.CodedUITestFramework.xml and put this XML inside it:

<assembly name="Microsoft.VisualStudio.QualityTools.CodedUITestFramework">
  <member name="T:Microsoft.VisualStudio.TestTools.UITesting.CodedUITestAttribute">
    <attribute ctor="M:JetBrains.Annotations.MeansImplicitUseAttribute.#ctor" />
  </member>
</assembly>

Restart VS and you're on your way!