Please remember that attachments larger than 100k can cause a major problem on the recipient's end. Here is how you send an attachment with Outlook Express. 
1.  Start a new mail message. 
2.  Address it and type a letter as you normally would. 
3.  Click on the Paper Clip icon. 
4.  That will bring up a dialog box that will allow you browse for the file you want to attach. 
5.  When you have found the file you would like to attach, click on it once and click on Attach. (On a Mac, click on Add, and then click on Done when you are done adding files.) 
6.  Click on the Send icon, and your attachment should send. 

If you have received a message that has a file attached to it, it will have a small paper clip next to it in your inbox. 
If you click once on the message, there will be a larger, yellow paper clip on the upper right hand side of the message view window (on a Mac, you will see a blue paper clip on the toolbar labeled "Attachments" with a pull-down arrow next to it). When you click on that paper clip you will get a list of the attachments with that message. Click on the attachment you want to view and if that file type is associated with a program on your computer, it will open in that program. 

If that does not work, you can double click on the message and it should open in its own window with the attachment showing as an icon in a separate section at the bottom of the message. Right-click on the attachment icon (click and hold on Mac) and select Save As. Then open the program you think it should open with, and use that to browse for the file.