I have pasted in my message some html code, but the layout of the page is different from what I was expecting.

If your HTML code is in a file, the fastest and safest way to load the content to your console is throught the "EMail->UPLOAD MESSAGE" menu. Please make sure that the HTML you are using is a valid file, with all the open and closed tags (HTML, BODY, STYLE etc.), and saved with the standard UTF-8 encoding.

If you don't have a complete HTML file, but rather just a portion of HTML code, you can add the code through the message editor enabling the HTML insertion mode with the button "<>".

You can also edit the default HTML headings clicking on "SHOW ADVANCED OPTIONS->CUSTOMIZE HTML HEADINGS".

With regard to editing the HEAD section of the document, please remember that it is important to use inline styles rather than styles defined in an external document or within a STYLE tag at the top of the document. That's because many email clients only support inline styling (e.g. define a table background color within the TABLE tag, instead of a background color set on the BODY tag).

With regard to images, make sure that you are using absolute URLs to either your MailUp console or another repository (e.g. your own Web site). For example:
<img src="http://www.myWebSite.com/images/picture.gif alt="My Image!">

For more information about creating a good HTML message, please see this article.