ASP filterexpression on ObjectDataSource GridView error 'does not contain a definition for _selecting and no extension method'

I am trying to add a search into a  gridview that uses an objectDataSource and I am getting the below error during a test run. 
I tried to follow the example on how to use FilterExpression using ObjectDataSource:
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.objectdatasource.filterexpression%28v=vs.110%29.aspx?cs-save-lang=1&cs-lang=csharp#code-snippet-3 'ASP.users_aspx' does not contain a definition for 'dsUsers_Selecting' and no extension method 'dsUsers_Selecting' accepting a first argument of type 'ASP.users_aspx' could be found (are you missing a using directive or an assembly reference?)
Here is my ASPX page:
<%@ Page Language= ...