Yes, Adventism is connected to Freemasonry, because the founders of Adventists were also Freemasons, and there are a lot of similarities. The biggest similarity is that both the Adventists and the Freemasons use a great deal of symbolism in order to make the connection between God and their faith. The symbols are also similar, and tie the two together.
There is nothing that I can find that would connect Adventism to Freemasonry. It appears that both movements have suffered under the common American delusion that anything just slightly out of phase with the mainstream must be sinister. The Adventists go to church on Saturdays and prefer to eat mostly vegetables. The Masons are about community service. Neither are that strange.
To establish a difference, we must understand that one is a religion while the other is like a club. Freemasonry does not require that you believe in a certain god to join, as long as you do in fact believe in god. Adventism is a sect of Christianity where there are requirements of what you believe in and how you believe in it. If there is an connection it isn't directly and they are no strong ties. I guess someone could be and Adventist and be in the Freemasons.
In order to find connection between Adventism and Freemasonry you have to dive pretty deep into conspiracy theories and uncommon trains of thought about Adventism and Freemasonry. Because of this, I have my doubts they are connected in the ways that are suggested. I don't think there's a secret society over all this, it just seems unlikely.
Freemasonry is a collection of often misunderstood bits and pieces of many other religions that the general public does not know very much about. Adventism doesn't have any more connections with freemasonry than any other religion, meaning any evidence is coincidental and anecdotal at best. Any religion that is on the fringe often gets treated with same suspicion as freemasonry and because of Adventism's birth in apocalyptic beliefs it is on the fringe.