install on version 5.x

Search by Page was developed very recently, and only supports Drupal 6. I don't have any plans to backport it to Drupal 5, sorry!

There's quite a lot of documentation on how to port Drupal 5 modules for Drupal 6, but very little about how to go backwards. It would involve:
- Creating Drupal 5 .info files (the format changed)
- Figuring out how to adapt the database schema stuff in the .install files into Drupal 5's old way of doing things
- Making changes in the Drupal core functions the module(s) call so they would be compatible with the version 5 core functions

If you'd like to take this on and provide a patch, I would be happy to make it available for others to use.

For Drupal 5, if you just want to restrict search to certain content types, try one of these modules:
http://drupal.org/project/search_restrict
http://drupal.org/project/faceted_search
http://drupal.org/project/views_fastsearch

If you want to search the entire text of each page, you could try integrating a 3rd-party search engine, such as Google:
http://drupal.org/project/google_cse
And if you do that, you'll probably want to tell Google what to index on your site (you probably should do that anyway):
http://drupal.org/project/xmlsitemap

Just a note that at this time, no one is actually working on porting Search by Page to work with Drupal 5.x (this person found a different solution).

If there is high demand, or if someone else wants to work on it, please leave a comment here.

At this point, I would say there is no chance I will be making a version of Search by Page for Drupal 5.x. Drupal 7.x will be released in a couple of months, and at that point, Drupal 5.x will no longer be supported. Sorry...