Find out the device name of the partition you want to mount. Check under /dev or open the partition in nautilus (Ubuntu's file browser/explorer) and run the "mount" command in terminal to identify the device name e.g. /dev/sda3
Go to Ubuntu dash (top right) -> Startup Applications
Add a new startup application as shown in the image below and you're all set! Command : /usr/bin/udisks --mount /dev/sda3
The drive will be mounted automatically when you login to Ubuntu. This solution works on Ubuntu 11.10 (Oneiric), Ubuntu 12.04 (Precise Pangolin) and Ubuntu 12.10 (Quantal Quetzal).
For further details, more advanced settings and system-wide mounting, check - https://help.ubuntu.com/community/AutomaticallyMountPartitions