Adding letter to reference

Hello, I'm trying to add a letter to the date of publication when there's more than one citation with the same author and year e.g. 2006a 2006b, but Mendeley erases the date and puts n/d instead. Is there any way to do it automatically? I tried what is recommended here and it didn't work: http://support.mendeley.com/customer/portal/questions/1306915-adding-a-letter-to-a-date-of-publication Thanks!