There are many options for sending attachments. You'll need to find the one that suits your needs the best.
You can use the Insert menu, and choose File Attachment. Then, you can browse to the location of the attachment, which will likely be in one of your personal folders. This is one of the more common ways to add an attachment, but often the most convoluted.
You can drag the file to the e-mail and it will attach itself automatically. If you have both the New Message window and the window that holds the file open, this is a good option.
You can also right-click the file's icon, select Send To:, and select Mail Recipient. This method attaches the files to a new email, but won't work with forwards or replies. This method only produces a new e-mail message.
You can also email from within applications, such as Microsoft Word or Excel. Generally, you'll find the desired option under the File menu. Although I can't say why, often this method fails. If you use it, look in the Sent Items folder once it's sent to make sure you actually sent the file and the version desired.
For related information visit: Microsoft Office 2007 – Office’s New File Formats