Here's how easy it is...
Custom Fieldsmeta box enter a name of CODE1 and your embed code as the value. Save this.
%CODE1%where you wish the embed code to appear.
And that's it - when the post or page is viewed
%CODE1% will be replaced with the code that you asked to be embedded.
This should get you started - for more information and advanced options please read the "Other Notes" tab.
Although this plugin works for both posts and pages for simplicity I will simply refer to posts - bear in mind that pages work in the same way.