HOW TO DO IT FROM THE TERMINAL
I assume you know the basic commands (sudo, cd, ls, mkdir, ...)
If you want to do it from the command line, you need to know one thing. List the files in the /dev folder. You should see somewhere in there one or more files named "sda" with a number after it. This is usually you primary hard drive. The number is usually the partition number ("sda1" is partition 1, "sdb5" is partition 5). If you have a secondary hard drive installed, it is usually listed as "sdb" with a number as with the primary hard drive. The goal is to identify which file represents your flash drive.
Now we need to make a folder to mount the filesystem to. There should be a folder in the root named "media". This is where other filesystems are usually mounted to. If you open it, you should see folders such as "floppy" (if you have a floppy drive), "cdrom" (if you have a CD drive" and possibly "sdb"+number folders, if you have multiple hard drives. You want to make a new folder (if it doesn't already exist) with the same name as the "sd"+letter+number file that you identified earlier. The naming of the folder is not mandatory, but (as far as I know) it is common practice to name the folder the same as the device. To make the folder, you probably need to do it as the root, so type the following while in the /media folder:
sudo mkdir <folderName>
Now to actually mount the filesystem. Run this command:
sudo mount /dev/<drive> /media/<folderName>
where <drive> is the "sd" file you found earlier and <folderName> is the folder you created earlier.
HOW TO DO IT FROM THE GUI
***I haven't actually run Ubuntu, but I have run Lubuntu, which is a different desktop interface. I feel confident that the procedure is similar. If I'm incorrect about this, I apologize.
Open up "Disks" (under "Accessories").
On the list to the left you should see your flash drive listed. Click on it.
You should see some info about the drive, a box that visualizes the partitions of the drive, and the current mount status.
If the box with the partitions, there should only be one filesystem listed, unless you gave it multiple partitions. Click on the partition that you want, and underneath it click the little right arrow (looks like the "Play" button on a DVD player).
If prompted to, enter your password.
Underneath, it should tell you where the filesystem is mounted.
I hope that you find this information useful.