XPath Builder location?

Probably a stupid question but I am new to Workbench. I have searched to see if this question already exists.

I'm in Process Designer view in workbench for a Process.

I double click on a Set Value item to bring up the Process Properties.

I expand Mappings and see the current Mappings.

I click Edit the mapping Data button and nothing happens.

I need to add a new mapping and searching Help seems to suggest this is done in the XPath Builder, but I cant find it anywhere? Do I need to install it as an add-in to Workbench?

Any help appreciated.