Hi Franz,
I don’t think I would personally consider Lookup stack to be an alternative to @isaiah’s Lockdown plugin. Lookup stack cannot really be used to password-protect entire sections of a website. I didn’t really design it for that job.
The purpose of Lockup is clearly presented to you here on my product page, along with working demo’s and a free demo version of the stack to download and try before buying:
So assuming you had a file named my file.pdf stored in http://arm.rathmair.eu/Formulare, you could easily setup the stack so that the word Formulare is the passcode.
Your settings would look like this:
[Click to see an enlarged view].
And the extension to /file.pdf
The word ‘Formulare’ becomes your passcode. Why? Becasue that is the word needed to complete the address and link the user to the PDF file:
http://arm.rathmair.eu/Formulare/file.pdf
Of course the stack is not going to function fully in edit mode. 
Like a lot of stacks, you need to press the ‘preview’ button in the RapidWeaver window to see how the completed webpage will look and function.
It sounds like you are very new to Stacks and RapidWeaver. As much as I appreciate your purchases and support for my work, I would urge you to perhaps please familiarize yourself first with the basics of using Stacks and RapidWeaver, like toggling between edit and preview modes:
As a side note, you should consider protecting your website with an SSL certificate (so all page addresses start https://) and use a convention of lower-case folder / file names. I only mention this because some web browsers (like Chrome) complain if they see forms or password input boxes on insecure websites.
There is an update planned for Lookup stack, but it doesn’t really change the functionality of the stack much. Mostly this free update will be adding some new style settings that existing users have asked me for.
I am experimenting with bringing the old Sentry stack back to RapidWeaver too, and re-releasing it as a major update. This would allow an entire webpage to be password-protected. Perhaps more suited to what you want to do?
But I have run into technical problems with getting this stack to preview correctly in RapidWeaver. Ideally I wanted to provide the new stack so that users have the ability to fully test their login system “live” in RapidWeaver Preview. So it is hard to put an ETA on this one currently, assuming these problems can be overcome.
I hope this information and the links help you.