For complex finds, I do not recommend that you have the user enter criteria while in find mode. It usually makes for much simpler scripting if you have them enter the criteria into global fields while in browse mode and then the script enters find mode and uses the data in the global fields to set up the find request(s) needed to find the records that you want.
Whether or not you set it up that way. The basic method is the same: Instead of Perform Find, the script needs to use Constrain found set with an omit request that specifies criteria for the records to be omitted from the found set.
Using your approach, your script can pause in find mode and after the user clicks continue or pressed enter, the script can do this:
Pause/Resume Script [Indefinitely]
Omit Records---> this step makes it an omit request
Constrain Found Set.
I knew there was an easy answer. Works like a charm!!!