You can’t because there isn’t.
But in the next update you will be able to ask for the current page of a document, or the current page of the content view (those are not necessarily always the same), and subsequently get the index.
You can then write something like:
tell application "PDFClerk Pro"
tell document 1
set a to the current page of the content view
set b to the current page
end tell
end tell
Ah, so you can, after all, you just can’t use it yet.