You need multiple Find requests to perform an OR search.
1) Enter Find mode.
2) Check your first checkbox.
3) Press Ctrl-N (or Cmd-N) to create a new Find Request.
4) Check your second checkbox.
5) Click "Perform Find" (or press Return).
Edit: Another option would be to write a script to accomplish this. You can create a global field, into which the user checks off whatever options he wants, then you create the Find Requests one at a time in a loop until you have as many as you need. Then perform the Find. All of this is done in a script, so all the user does is click "Find", then sees the option list, then clicks "Go".
Perhaps you're not understanding how checkboxes work? it's a FIELD with return-delimited values, in the order they were "checked". place the same field on a layout and format one as checkbox, one as plain. That will help you see the "list" you create by checking & unchecking.
That being said, If you put "X¶Y" into a field to find either, it will be looking for "X", a return character, AND "Y".
If, on the other hand, you put "X" in the city field, make a new request and put "Y" into the other field it will find "X" OR "Y". Add the amount you need in each "request" or use Constrain Found Set to find within the found cities those with the correct amount ("<20000").
It take me 10 seconds, too.