Hi,
Welcome to Slackware!
For future problems you can look at
'
SlackwareŽ Essentials' and '
SlackwareŽ Basics' which are good books to reference for problems or solutions to other possible problems.
Looks like you are attempting to install to a logical on the extended partition. As long as the partition is prepared with the filesystem of choice then I see no reason to worry.
If you use the 12.2 boot cd1/dvd you will need to format the partition so the filesystem is created. '30GB' is more than enough space for the install. This can be done within the 'setup' or from the 'cli' with the 'mkfs' front end.
Once you boot the system with the install media you can do a 'fdisk -l' to show the disk information. Please post that?