I am in the same boat here. Trying to get PayFlow (which has been purchased from Verisign by PayPal now) integrated into a self-built cart/checkout system.
I have very little experience with anything outside of straight PHP/MySQL-based things.
I will be contacting someone at PayPal to see if there is any information on how to get it setup on the server. Although I'm not sure if this will be possible without a dedicated server. Again, I don't know very much.. literally just enough to be dangerous. If I discover anything, I'll be sure to post it here.
I just spent the last, however long it has been since I made this post, trying to get PayFlow installed. Hours later, I'm frustrated, just sent a looooong detailed e-mail to PayPal asking for documentation that windows users (me) can follow, and PayFlow isn't close to working at all.
This is bad, PayFlow appears to be the only service that does what I need (seamless integration with whatever checkout system you have, in my case a self-built one that ties in with an inventory/accounting system I have as well, so going to a third-party cart system is out of the question.)
I'm giving up on this for right now, I have some really tech-savvy friends coming back into town next week, so I can probably get their help at that point if PayPal just tries to sell me on their integration-assistance service.