You must first set up the exchange rate provider you want to use, and AX comes with 3 providers ready to use (More can easily be developed) in General Ledger / Setup / Currency / Configure Exchange Rate Providers :
Once this is done, General Ledger / Periodic / Import Currency Exchange Rates allows you to choose which Exchange Rate Type you want to import the rates for, and using which provider.
You can of course set one up for each rate type, and even schedule these to import periodically using the standard AX batch feature
Once this has run, your rates can be reviewed in General Ledger / Setup / Currency / Exchange Rates :
This illustrates very well how Dynamics AX Web Services integration contribute to make AX 2012 powerfully simple.