I thought there was an option in the partition dialog to allow you to install Mint on available free space. If it is available, and you haven't tried that option suggest you give it a go.
If you're up to the task (sounds like you are given that you have already created unallocated space on your hard drive), my suggestion is to pre configure the partitions that you want to use for your Mint install. After this is completed, use the "manually configure partitions" in the installation. This option takes a lot of the guess work out of the installation process (my opinion).
Linux'd since 2005 Linux ID 422356 Ubuntu ID 15015