Enable Hidden Button in CRM 2013 Using Ribbon WorkBench

Open Ribbon editor and select entity on which you want to enable button. In this example i enable Export to Excel Button on form.

Click on Command Bar and select button then expand command. Now right click and select Edit Display Rule

Now <Remove "Mscrm.HideonCommandBar" Rule. and other rule IsCore to "True"

This will Display "Export to Excel" Button on Competitor form.

To disable and enable ribbon button on change of Field Value using ribbon editor

Follow those steps:
1.      Click on the button
2.      Expand command
3.      Right click on command and select edit enable rule.
4.      Then click on the rule and press edit button.
5.      Write the field physical name in field section
6.      And also give invert (true or false) as your condition.
7.      In value section write the value on the basis of which button enable or disable.

Below Video is good :

http://www.youtube.com/watch?v=OE1CVePTeTc

Disable Lead Ribbon button using javascript in CRM 2011 and 2013

var x= top.document.getElementById("lead|NoRelationship|Form|Mscrm.Form.lead.ConvertLeadQuick-Large");
x.disabled = true;

Refresh Ribbon using javascript in crm 2011 and 2013

Xrm.Page.ui.refreshRibbon();