how can i get my rads hot?

hi guys hope some-one can help.
i've got a Worcester 24 cdi installed which works okay,the downstairs
rad is working okay but the upstairs rads only the bottom inch/inch
and half are hot which makes them as useful as a chocolate teapot,
i've been i told it could be that theres not enough water in the
system,but its been like that since it was installed,weather the
people who installed it did it expecting a callback for extra
bunce,but thats what you get when you use (a friend of a friend), i
dont know.
anyway back to the matter in hand, i'm not an engineer but i,m no
muppet either, i've seen a few diagrams but theres nothing about how
to get more water into the system ( if thats the problem)
If some-one could give me some straight forward directions i should be
able to cope
thanks in advance for any help

hope some-one can help as i,m double duveted at the moment :-)))