Certificate's private key grants access to the application pool user

• Open the certificates MMC.

             Download file hear and open it or manual Step below.

• Open MMC
• Click File –> Add/Remove Snap-in…
• Choose Certificates and click Add
• Select Computer Account and click Finish

• Check if the certificate is available in the "Local Computer\Personal" cert store.
• 
  
• Right click on the certificate and choose "All Tasks --> Manage Private Keys" and "IIS_IUSRS" or other user or app pool account that the IIS 7.5 app pool is using (ApplicationPoolIdentity).

>> Click on Location and select top most item.

>> Then write "IIS_IUSRS" in object box. then click ok.

Enjoy........

Source: http://bartwullems.blogspot.in/2012/02/grant-iis-75-access-to-certificate-in.html