As you noted most of our products are DW extensions and stand alone solutions. The new buttons for PayPal addin for Expression Web 4 is the only add in that we currently offer for ExWeb 4.
Once you download the file you need to open your ExWeb and go to Tools > Add ins then click on install. Just browse to the file you downloaded after this.
Once it is installed you bring up the pane from the WebAssist menu.
How to install it and use it are covered in the two videos that we have for this product. The installing video can be found on the Expression Web Add In page at the top where it says Video: Understanding Add-Ins. Here is a link to this page:
The other video we have that demos some usage of the tool can be found on the 'Payment Buttons for PayPal' page in the feature tour, here is a link to this page:
Please let us know if you have any questions or if you encounter any problems.