Which revision of ShopSite 11 do you have?
The iframe method was added to ShopSite 11sp1, os if you have the original ShopSite 11.0 you won't have the feature.
To get the code for the iframe go to Merchandising > Order Anywhere, select a product and click Show Code. On the screen that displays there should be 3 buttons, Show Html, Show URLs, Show embed. Click the Show Embed button. On the left will be a window with the code to paste to your page, on the Right is a preview of what it will look like. Note that it uses a custom template which is very basic. You may want to create your own template (using the default one as an example) and add what ever features you might want. I don't know if cross-sell items will be accessible to this code but I think that subproducts probably would so you could also look into using subproducts as cross marketing. Note that the code on the left includes the template name so if you change what template is used you will also need to change any Order Anywhere links for that product.