Step 6 – Click on Open > Complete after browsing to your certificates. Exchange 2010 may display an error message like "The source data is corrupted or not properly Base64 encoded." You need to proceed by ignoring that message.
To refresh the certificate, press F5 and make sure it says 'False' under 'Self-Signed'. If it still displays 'True', you may want to check for incorrect certificate selection or a request generated for a different server. To fix this problem, create a new CSR on this Exchange server or re-issue the certificate.
Step 7 – Return to the 'Exchange Management Console' to enable your certificate for use. Now, click on 'Assign Services to Certificate'.