There is a whitepaper available in the downloads area of this site that you may find helpful. 

http://community.pmail.com/files/folders/whitepapers/entry41852.aspx

Review it then post back with specific questions. 

Hello,

My old PC died, but I have a backup of the PMAIL folder. It is a very old backup 2007 but I need to get at some emails which are only in it.

I have installed a newer version of Pegasus onto my new laptop running Windows 8, which I had already copied the old PMAIL onto, but it has not recognised any of the previous data.

Does anyone know how I do that?

If you only need to access messages from the backup then the best approach is to do a clean install of the most recent Pegasus Mail onto the Win8 machine.  Be sure to install into the default location of C:\PMAIL (it will not work if installed anywhere in the \Program Files folders on Win8).  You could then access the messages from the backup by adding that mailbox directory as an added folder to the Pegasus Mail folder list. To do this, make the folder list active the go to the 'Folder' pull-down menu and select 'Add mailbox to list'.  In there you select 'Directory path' and enter the path to the mailbox directory of the backup.

If you need all content from the backup then you still need to start with a clean install of Pegasus Mail but will need to copy files from the backup mailbox directory to the new mailbox directory being careful not to overwrite any configuration files in the new mailbox.  Han's guide to Pegasus Mail filenames and file-extensions is a valuable resource for this and can be found here:
 
http://www.vandenbogaerde.net/pegasusmail/pf_pmfiles.html 

I need help with the same issue!  I think I did this once before and just copied over the files (two files per folder) to the new install. Not totally sure how to proceed though!

It's a pity my PMAIL 4.60 crashed and I had to go to 4.70 but it seems more stable so far!