Crm 2011-Disbling Ribbon button based on Form field value

1.Create a new button using visual ribbon editor 

2.Open Enable rules tab ,click add button and choose ValueRule as follows

 

3.Enter the field name and value as follows

Note: For OptionSet fields,value is the OptionValue ,not the option Label 

For more details

http://howto-mscrm.blogspot.in/2011/04/how-to-series-5-how-to-use-valuerule.html