contact the manufacturer... should be able to get a replacement section. Its not uncommon to bend one during installation or removal if you are not careful.
As for securing it... I use a big thick heavy chain and a big masterlock around the tree. I tape up all of the ladder clips to prevent them from jingling, and that makes it more slightly difficult to remove. If you were really concerned you could lock each ladder section together with chain/cable locks. You could also do what another member here did... installed a CONCRETE FOOTER and bolted his ladder to it!!