Authorize.net creates your Simple Checkout button in the form of a form tag - and that cannot be used on a DNN page. This method of making it work is a little convoluted but does nevertheless work.
Create a html page, put it in the root of your site, name it whatever you like. I suggest "PaymentPage.html"
In the HTML of that page, enter the following line in the body section, this code will break the user payment page out of the frame it is in.
Then include the code for the Simple Checkout button like so:
<form> name="PrePage" method = "post" action = "https://Simplecheckout.authorize.net/payment/CatalogPayment.aspx">
<input type = "hidden" name = "LinkId" value ="xxxxxxx-xxxxx-xxxxxx-xxxxxxx" /> <input type = "submit" value = "Pay Now!" /> </form>
Save this file.
Then go back to your DNN page and in an html module, add an iframe that points to your html page similar to this.
Now you should have a paynow button showing on your DNN page. The paynow button is actually coming from the form tag on the PaymentPage.html file that is showing in an iframe. When you click the button. The base target tag breaks you out of the iframe while it takes you to the Authorize.net Simple Checkout page. Success!