First, hooking up something with no over-current protection at all is right out. It has to go on a breaker.
That said, it is not critical that it be a dedicated breaker. They also tell you it needs to be in the top left (nearest the main) breaker position, but again, that's only ideal, it's not critical.
I simply install it on existing branch circuit breakers, and I double-tap them because Pushmatic breakers allow that. You can wire-nut to a pigtail. Also I don't bother handle-tying if the SPD is going in the main panel, because the cross-connection is obvious to anyone servicing the device.
It is important to NOT place an SPD on a GFCI or AFCI breaker. It is also important to not put a refrigerator on GFCI protection.
No need to install at the main panel
It may be marketed as a whole house surge protector, but that does not need to be installed in the main panel. Wire length has a small spike-attenuating effect, which is why they want it in the main panel. However if you are using multiple SPDs to protect (or suppress) various appliances, the closer you are to the appliance, the better.
Keep in mind your appliance may be a creator of noise and spikes; they don't all come from outside your house. In fact, transformers are fairly good at attenuating spikes, so unless there's industry on your transformer, I would expect most spikes to come from yours and other residences.