You can add flash elements to your pages by editing the HTML code.
Click the HTML button. That brings up a window for you to insert your code. If you are inserting a video from youtube.com or some other site, you simply copy the code that they give you on that site and insert it into the HTML by pasting it in.
Your paste section should look like the image above. you want to make sure that then width of your video is less than 500px, otherwise it will stretch out your page.
You can also link to a flash file by clicking the INSERT FLASH button and pasting in the address of the flash. This file will end in .swf and it should be one that is already uploaded to the internet somewhere.