Make Linux Mint appear as ubuntu

Most likely the installer is checking /etc/lsb-release. In Linux Mint, the same file for the Ubuntu version it was derived is under /etc/upstream-release/lsb-release. To fool the installer, just replace the former with the latter (although you probably want to back up the file first). In a command terminal you can do:

sudo mv /etc/lsb-release /etc/lsb-release.original
sudo cp /etc/upstream-release/lsb-release /etc/lsb-release

At some point after your install is done, you can restore the original with:

sudo mv /etc/lsb-release.original /etc/lsb-release