you're using. My answer is based on Outlook 2010, but it will hold good for most recent versions.
When you type a new email you'll see a 'from' button at the top left - just above the address field. Click the small arrowhead and you'll be presented with a list of all active email accounts; select the one you want to send from.
As far as inbound mail is concerned, Outlook will lump it all together in your in-box unless you tell it to do otherwise. To sort incoming mail by account you can create a separate folder in the default inbox. Call it (name you choose) inbox, and then create a rule telling Outlook to place all inbound mail addressed to the account in the folder you've created.
If you need more detailed help in setting all this up just ask.