Happy to say that this is a new feature in FileMaker Pro 11 (as a result of requests such as yours).
In FileMaker Pro 11, you can "Use variables in scripted find requests – You can include local and global variables in scripted find requests that you create or edit."
It is easy. Here are the script steps:
Enter Find Mode
Set Field (with your variable)
If it doesn't work as expected, go into Debugger and Data Viewer and watch your fields as the script runs. If you don't have Advanced, then type 'Command- R' (Mac) or 'Control-R (Windows) AFTER running the script. Then you can see what was entered into the field via the script in Find.
This was not new to filemaker 11. I've been doing this in filemaker 10 from day 1.
As Mike Williams points out, this works if you enter find mode and then set field with the variable,
but I was hoping to use the perform find script step and build the variable into the Find and Omit dialogues and that didn't work for me.
The script posted by Mike was possible in FileMaker Pro 10.
However the new feature in FileMaker Pro 11 is that you can specify a variable in a criteria for a stored Find request. This means that all you need to do now is:
Perform Find (with stored criteria using variable)
So pirxx, I believe this is what you were looking for.