Do you have time between the time the user clicks the DWA link and when the user performs the action you want to trace ?
If so, use the requestor dropdown on tracer BEFORE clicking DWA link to see what the list looks like. Then click the link and look again so you can see how the list has changed and thus figure out which requestor to choose to trace. (Using this dropdown at all is called "remote tracing").
The above depends on the tracer's ability to update the requestor list after the tracer starts if some requestors come and go. I'm not 100 percent sure that happens. If not, you'll have to start tracer a second time after clicking your link.
To add a bit more detail to what's been said, until the user clicks the DWA link, there is no requestor to trace. Once the link has been clicked, you can find and trace their requestor via remote tracing (as Eric explained). If the bit you're trying to debug is in the initial click, I don't know of a good way to do that. I'd probably start by adding log statements to the activity to confirm it's running/provide some insight into what the data looks like. You may have to be iterative as you zero in on what's not working the way you'd expect.