darkstar is the default hostname (i.e. name for your machine, if you don't choose one). During the installation, you will be asked to set a root password, so to log in, you enter "root", followed by your password. You'll want to create a regular user account, for everyday use, though. This can be done with the command adduser.
As for choosing the partition to install on, well, have you run (c)fdisk to create a Linux partition out of the free space left on the disk after shrinking existing partitions? If not, you'll need to do this. Slackware's installation will detect Linux partitions on the disk and ask if you want to install on those. It will also detect Windows partitions and add entries to your /etc/fstab, if desired.