The issue is most likely the wrong file path.
Go to the directory Users\xxx\Appdata\Roaming\MIRC\.
Look for the mirc.ini file. Copy it and paste it in the same folder as the mirc.exe you are running.
This will make it so the directory for your files is the one mirc.exe is in. There won't be any question about the file path. Now you can enter your popups.ini in the script editor, or copy and paste your complete popups file to the folder mirc.exe is in.
Once you can verify the popup file is loaded it should work fine.
If you like you could also put the popup code in a menu bracket.
menu channel {
;example:
Popup:popup command
}
You would put this in a regular script file. (alt + R)
