The MBR stores a list of the primary partitions and a very small program that
is executed by the BIOS. There are plenty of tools to manipulate it using
GNU/Linux including “fdisk”, “cfdisk”, “parted” and
“gparted”. However, as already pointed, your problem seems to be that you
haven't chosen a partition to mount in “/” when installing.
Be careful when manipulating partitions and the MBR. A mistake can easily
make your system unbootable if you misconfigure the bootloader (GRUB). You
may have to attempt file recovery or find the partition extents if you delete
partitions from the MBR; on the other hand, if you shred (set all bytes to 0
or random data) you will lose the data contained therein. That is not to say
that you must be scared, only cautious and check what you are going to do
before doing so.
If you are installing on a hard disk which you intend to use entirely for
Trisquel, then just chose the option “use all the disk space” (I don't
know the exact wording) and select the right disk instead of manually
configuration partitions. Putting everything in a single partition plus swap
is likely to require the least maintenance to you. If you want something more
flexible, you may use LVM (and you need the netinstall image to use it), but
I recommend only if you want to spend time learning that aspect of system
administration.
It is unlikely that you will lose information due to a bug when installing,
but it's always a possibility[1] that add to the chances of plain user
mistake and therefore it is a common and good recommendation to perform a
backup of your important information before installing an operating system,
and also to do so periodically.
[1]: until informatics reached the state of development when the relevant
software is mathematically proven correct to a specification.
