Greg - First thing; Paypal API
In your paypal account click 'profile' and then on the next screen look for the link to 'API Access', you then get another screen where you select 'view API certificate' then next page (!) you select the 'request API signature' check box and then you should get it...

as for the rest of your Q's, I'm as stuck as you are. seems like a fair few of us are at the moment, not sure if anyone has been able to find a solution - I haven't figured it out myself!

I got this error too, the blankness... everything looks amazing, really cool work and all but all pointless... I bet it some silly little thing, anyone any ideas ?

Not sure honestly, a blank page normally means something in the php page generation process got hosed up. Check your cgi logs for errors.

However, while I'm giving advice on this, you should probably upgrade to the stable 1.2.6 (1.2.7 is around the corner). This hack/plugin is based of 1.2.4 and is a bit old, on top of that the author seems to have been inactive on this.

If you goto 1.2.6 (or the nightlies or upcoming 1.2.7) take a look at http://www.zenphoto.org/support/topic.php?id=6424 the old plugin that this author created was picked up by another user and improved upon to use the paypal cart feature etc.