You're saying many conflicting things... and you seem a bit over your head in this, but here goes.
If Mint installer does not give you an option to automatically create a partition for itself (which it doesn't, for you, if I got it right), you should choose to do so manually ("Something else").
The steps you need to take are:
1- Split a chunk from one of the existing partitions (by using partition resizing tool provided) and size it properly (give it at least 20GB, it will work with less but will bite you eventually)
2- Create an ext4 partition out of the unallocated chunk created in the step above, but leave a bit for swap (see below)
3- Create swap space out of the remaining unallocated space
4- Choose "/" as mount point for ext4 partition
5- Proceed with installation
NOTE: If you make a mistake, you can easily destroy your existing partitions loosing all data you have on them. This can happen even if you do not do anything wrong, but is unlikely. Backup everything of importance and triple-check everything before resizing/modifying partitions.
NOTE 2: I've seen it many times that people report problems when dual booting with Windows Vista/7. Something about boot loaders. On the other hand, most users have no problems at all. Make sure you read a little on this topic (search the forum) beforehand to see if it concerns you or not.
I hope I didn't forget anything, but if I did, someone please chip in.