1) Enable "Auto-Return" and "Payment Data Transfer"

Log in to your Paypal account.
Go to Profile.
Click on Website Payment Preferences
Turn Auto Return to On (see image below)
Set Return URL to http://YourSite.com/dap/dap-thankyou.php
Scroll down further to the Payment Data Transfer (optional) section
Set Payment Data Transfer to On
Scroll all the way to the bottom of the page, and then Save the changes.
Now come back to this same page just like you did before (Profile > Website Payment Preferences)
This time, simply scroll down to the Payment Data Transfer (optional) section
Now you will see a long, ugly string of text next to the Identity Token field (see image below). Copy that string.
Go back to DAP Dashboard > Setup > Config > Payment Processing > Paypal Identity Token field, and enter this string there, and save it.

2) Some More Config Settings (Defaults)

You don't need to change any of the other settings, but they are explained here anyway. You will find these under DAP Dashboard > Setup > Config > Payment Processing section.

3) Generate Buy Button from within DAP

You can generate the button for Paypal for your product, on the DAP Dashboard > Payments/Coupons > Generate Paypal Button page.

4) Publish generated Buy-button generated on your Sales page.

That's it!

Sales Flow

Here's how the flow works now:

Prospect clicks on buy-button your sales page and is taken to a Paypal checkout page at Paypal.com
They purchase your product and complete the payment
They are initially redirected to the "Auto Return" url from Step 2 above
DAP captures customer email, name, product name, etc
DAP creates member account for buyer on your system, and gives them access to the product, will send out the Welcome-Email from the product page, etc
It auto-logs them in to your membership site - no need for member to wait for Welcome email just to get their login information (that email is sent anyway, and will be waiting for them in their inbox)
DAP now needs to know where to redirect the user to.
a) If Product-level Logged-In URL is set in DAP for this Product that they just purchased, then it redirects them to that URL
b) If no Product-level Logged-In URL is set, OR if the buyer has access to more than 1 product (if they had previously purchased another one of your products, for example), then it uses the Global "Logged-In URL" setting from: Setup > Config > Advanced > URL to which user is redirected to right after login

2) DAP Generated Buttons For Paypal With DAP 4.0, we have added support for DAP generated buttons for Paypal. You no longer have to generate buttons within Paypal.