Open the active.asp file from the poll mod zip file (call it file 1), in your text/html editor, then open your active.asp file from your forum (call it file 2), then look for the poll mod code in file 1 and copy the code into file 2.
The poll mod code is marked out in the files so you can see what exactly you need to add to your files.
Alternatively you could use a file comparison tool. I use Beyond Compare (http://www.scootersoftware.com). You can use it for 30 days before you buy it. But if you just need to install your mod, it is a great tool to use. You can try searching for free file comparison tools.