How to use check box when use with Selenium IDE in a data driven XML?

Trying to run an automated test where the sometimes the IsAfterHours checkbox needs to be either turned on or off.

The XML data driven file have been created:

<testdata>
   <vars vClick1="id=IsAfterHours" />
   <vars vClick1="" /> 
</testdata>

the code use to test this feature

forXml    | file://... |
open      | URL |
click     | ??? |  ***How to control the checkbox from XML?
endforXML | |

Please help... Thank you in advance!