I see what you mean now. There should be at the top of the new message screen several lines showing things such as the To: line, the From: line, the Reply-To: line, the Subject: line etc.... You should be able to click on a drop down list on one of these which will show you all the accounts you have and let you set one to send that mail from.
If you don't have the option for that, then I have no idea what you've done! Don't use Outlook myself, and am a bit rusty on OE from memory as I've been using Mozilla Thunderbird for quite a while now.