I just found this question because I was having the same problem.  I have read through many tutorials suggesting the use of this PagedList object, but none of them said anything about adding anything to the web.config file.  This is what I did in order to make this work.  I used some of the info from the comment from @BuildStarted.  I am using VS2010.
Add this to the web.config file:
<add assembly="PagedList.Mvc" namespace="PagedList.Mvc" tagPrefix="plmvc"/>
So it should look like this (snipped):
<configuration>
  <system.web>
    <pages ...>
      <controls>
        <add assembly="System.Web.Mvc, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" namespace="System.Web.Mvc" tagPrefix="mvc" />
        **<add assembly="PagedList.Mvc" namespace="PagedList.Mvc" tagPrefix="plmvc"/>**
      </controls>
    </pages>
  </system.web>
</configuration>