By default older Mac computer models have driver issues with the Broadcom Wi-Fi chip. Most instructions online (such as this one from the official Ubuntu docs @ https://help.ubuntu.com/community/Macmini5-1/Precise) suggest to use sudo apt-get and install these packages from the PPA.
BUT you don't have Internet so you can't use apt-get! Instead you can use a different computer to download the packages, then transfer them over.
In the instructions below, you will need to replace sid in the download URL's with the proper version of Debian your version of Ubuntu/Linux is using. To find which version you should use, you can view the chart at https://askubuntu.com/questions/445487/what-debian-version-are-the-different-ubuntu-versions-based-on#445496. If you're using jessie for example, then replace all sid with jessie in the links below.
- Download
b43-fwcutter@ (*