Make sure you've downloaded the package android.support.v7. Then, Import the whole appcompat lib (/extras/android/support/v7/appcompat) as a library project to your workspace and then add to your main project as a library:
Here are the steps.
First check if android-support-v7-appcompat is already in your workspace. If it is go to Step 2
Step 1:
Select File > Import.
Select Existing Android Code Into Workspace and click Next.
Browse to the SDK installation directory and then to the Support Library folder. /extras/android/support/v7/appcompat/.
Click Finish to import the project. For the v7 appcompat project, you should now see a new project titled android-support-v7-appcompat.
Step 2:
Right-click on your project -> Properties
In Android->Library section click Add
Select android-support-v7-appcompat -> Ok