Get with it: to become knowledgeable about the current or latest trends, fashions, or ideas, as in She just turned 60, but she's still very much with it, or Get with it, Dad, that kind of razor hasn't been made for years. [Slang; 1920s]
The American Heritage® Dictionary of Idioms
To "get with it" is a somewhat informal idiom meaning to catch up on a situation, understand what's happening or be up to date with trends. It's a BIT old-fashioned but not extremely so. I think I would only use it being silly/ironic!
JOHN: Hey, I need to play some music. Do you have a tape player or something?
ALEX: Uh, I have an iPod! Get with it! :roll:
*At the office*
MARK: Can you believe what the board is asking us to do?
LISA: They act like we still have our 2006 budget to work with. They need to just get with it, you know?