i don't think they used an additional adhesive. they just used the putty that comes with the power meter. so when it dries, it hardens. the install looks like a pimple on the bike. a mound of putty, with the magnet at the top. i've heard of some people gluing the magnet to the frame.
i've actually had one recommendation to use 3M double sided tape.

the pull on the magnet is actually pretty strong. so i'm not afraid that i will ever lose the magnet b/c it will stick to something on the way down. that usually happens to be my chain.