The office is very easy to find, especially with their bright orange street sign. Zach was the employee who helped check me in, and he was amazing and made my experience very welcoming and enjoyable. It was like we were old friends catching up.
The parking garage is across the street and can be difficult to find if it's your first time and you don't know Seattle. Zach (or other Sixt rep) will tell you before you leave the office, but in case you want to be overly prepared: when you exit the rental office, turn right and walk to the stop light, make a left on Pike Street, and after you cross the crosswalk the garage entrance should be about 80 feet on your left.
I received a Benz A-series, which was clean, had few miles on it, and drove smoothly. The Benz itself had some questionable design features (e.g., where the heck is the radio scan button? And why do the doors "click" and halt so many times when you open and close it?), that have nothing to do with the rental company. But I stated them just in case you wanted to ask the rental attendant about these features. If you do ask, please let me know here 'cause I'm so curious :) Happy trails!