Hey Paul,
For preconditions, just add those conditions in prose in the preconditions text box (open the use case editor for the use case, click on details).
For triggers (invoked by some set of conditions that occurs within the use case) I'd use an extension, and then link that extension to a separate use case that embodies the trigger as a subfunction (I find it helpful to add the conditions invoking the trigger into the precondition of that subfunction use case). If there's a particular step in the use case where that happens, you can add the extension to that particular step. Otherwise, add it as an anytime extension and describe the conditions that trigger it.
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