The second piece of Metro: Last Light DLC should be available to everyone on the PC today. It’s proliferating now (Tim reports that a restart of Steam made it show up for him,) so if the game is claiming you don’t own the ‘Tower Pack’ yet and you think you should, there’s no cause for alarm.
It is, of course, part of the ‘Season Pass’ for this game, but can also be picked up as a $5.00 USD standalone purchase. In it, you’ll be fighting your way up a combat simulator tower, taking on waves on increasingly tricky foes on each floor.
Might be an idea to hold off on that though. A fair amount of players are noticing that trying to launch the Tower Pack simply causes the game to crash. Our own Tim McDonald is one of these very people. “This is a wonderful addition to the game, because previously there was no way to quit directly to the desktop from the chapters screen,” he told me, earlier.
[Update]: It now seems to be functioning without crashing, at least according to Tim.