How do I add the Multnomah County tax form to my return?

Yes.  The tax forms that you're looking for (Portland /  Multnomah County Oregon Business Tax, Tri-Met) are available as part of the TurboTax 2016 Oregon module, along with the Lane County LTD form, and the Portland Arts Tax form.

However, in order to complete any of these, the TurboTax user first has to "activate" them by going through the Oregon module interview.  In fact, there's a special place titled "Other Forms -> Additional Forms" where you need to check, or select, a box in order to add the specific Oregon local tax form(s) you want.

Or they can be found online at Multnomah County tax forms    Assessment and Taxation Forms

2016?

I'm using TurboTax online, so this doesn't appear helpful to me.  I'm going to fill out the form by hand and won't be looking for any further help with this.