Bring up a terminal, under Applications, Accessories click on terminal.
In the terminal window enter the command "cd /dev" with the quote marks.
Now enter the command "ls -l tty*" if you have a serial port.
I may look something like this:
crw-rw---- 1 root dialout 4, 64 2012-06-07 17:13 ttyS0
crw-rw---- 1 root dialout 4, 65 2012-06-07 17:13 ttyS1
crw-rw---- 1 root dialout 4, 74 2012-06-07 17:13 ttyS10
.
.
crw-rw---- 1 root dialout 4, 86 2012-06-07 17:13 ttyS22
crw-rw---- 1 root dialout 4, 87 2012-06-07 17:13 ttyS23
crw-rw---- 1 root dialout 4, 88 2012-06-07 17:13 ttyS24


The way to read this is looking at the left side it tells us it
is a character device with read write and no execute privileges for
the owner of root and group of dialout for the devices ttyS0 through
ttyS31 on my Mint system which is a Ubuntu spin off and nearly the same.
There are commands to add yourself to the group dialout to give you
permission to use the ttyS* ports. In my home directory if I do the
ls -l command I see my owner as marty and group marty so I would have
to add marty to the dialout group. This is what part of my group
file under the /etc directory looks like:

kmem:x:15:
dialout:x:20:marty
fax:x:21:
voice:x:22:

In my terminal window I would enter commands like.
sudo vi /etc/group

Then go the the line with dialout and go the end of the line and add
you id after the colon. Then save the file. Use you favourite editor
if you are not use to using vi.
There is a command tool you can use too but I am more comfortable using
the manual editor myself.