When you attempt to use Pages '09 v4.3, ensure that it is the only version of Pages that is running. All releases of Pages v5 do not natively support Mail/Contact/Data merge. On a Mac with both Pages versions installed, Pages v5.6.2 will be the default launching Pages application, unless Pages '09 v4.3 is already running.
Any Pages '09 document that has features unsupported in Pages v5.6.2, and that you allow to be converted into Pages v5 document format, will lose those unsupported features (e.g. merge fields). Exporting that document back to Pages '09 format will not restore the lost functionality.
Personally, I do not use Pages '09 v4.3 for any form of Merge activity, but I thought that some of the Merge panels, when I was testing, looked different, or were incomplete after upgrading to El Capitan.
There is a Pages for Mac Support Community.
You might be better served having the hosts move this post into that community.