JavaScript Void(0)
You can use the following examples to enable/disable buttons that have been disabled in the HTML source. Obviously, you have to adjust the JavaScript to fit your specific case.
javascript:document.accrual.elements['Add'].disabled=false;void(0);
javascript:document.chartField.elements['Update'].disabled=false;void(0);
Here's how it should break down:
javascript:document.FormName.elements['Name of Form Element'].disabled=true/false;void(0);
The void(0) is what allows you to execute the JavaScript without refreshing the page.