u dont. U just press the "select" button on your gamepad when inside the menu and poof! you are in battlelog seeing your friends.
You have to link your PSN ID and your Origin accounts and then you will be able to access battlelog from in game. Here is how to link your accounts:
Step 1 - Go to www.ea.com
Step 2 - Click on 'Login' in the upper right corner, if you are already logged in, log out.
Step 3 - Now the login form pops up, ignore the textfields and click the PSN button below
Step 4 - A new window pops up that allows you to login with your PSN account.
Step 5 - Now that the EA Server knows that you are the owner of this PSN account and if you already have an EA account you can now login to connect both accounts. In case you don't have an EA/Origin account yet, simply click the link on the right side to create one.
Step 6 - start BF3, at the BF3 main menu right click the d-pad to open your battlefeed and log in with the origin account that you used when you installed BF3.