The password it wants is the admin@system-domain, which is the password you typed in when installing VMware SSO during your vCenter 5.1 install or upgrade. If you don't know the admin@system-domain password, the only supported fix is to re-install SSO (see Option #1 below). You may also consider trying one of the other options. Each option below is mutually exclusive.
Option #1 (Re-install SSO)
Repointing and reregistering VMware vCenter Server 5.1.x and components (VMware KB 2033620)
Option #2 (glean password from db user)
If you set all of your passwords exactly the same during your SSO install, you may get lucky and learn your admin@system-domain password by executing the following command from an elevated DOS prompt in Windows (the first line is the command, the second line is the response):
C:\>findstr db.pass "C:\Program Files\VMware\Infrastructure\SSOServer\webapps\lookupservice\WEB-INF\classes\config.properties"
db.pass=Password01
In the example above, the password for the database user is returned as "Password01". Assuming at the time of the SSO installation the technician used the same password for everything, then and only then would the password for admin@system-domain also be "Password01".
Option #3 (DB Hash Technique)
This one gets a bit geeky and is not for the timid. This requires some SQL experience. The easiest way to do this is using the query (get it here) so kindly provided by memaad. This unsupported work-around is based on the german die-shubis blog post from here (or translated here).