Have you primed it? If you are a first time exturnal owner I'd guess you haven't...
Step one, remove all water from the filter and hoses. Position the inlet and outlet sets and plumb up the hoses between them and the inlet and outlet ports on the filter. (Check they are plumbed to the correct ports. I've set many up now, and still frequently make that mistake

) Now, check that both the inlet and outlet valves are open. Now you are all set to go.
Keep the filter switched off at this point, or you will run it dry and potentially burn out/damage some of the components, and that's expencive and obviously not good... At this point, you prime the filter...
Take the outlet pipe off the back of the tank and suck for a few moments. You should see water in the inlet pipe go up and over the side of the tank, and then a sound of running water is usualy heard as the filter starts to fill. At this point, place the outlet pipe back on the tank side, and go make a cup of tea (very important that last step

) One the tea is made and drunk, come back to your now full and "primed" filter, and check it for any signs of leaks (it shouldn't leak, but being 2nd hand it is a possibility). If all looks good, plug it in, and after a bit of spluttering, it should start to pump water

If it's moving water, it is all good and you should not need to touch it for 6+ months...
On a "tips and trick's" point, make shure you apply Vaseline to all seals every time you clean the filter out. When in constant contact with water, rubber hardens up and eventually snaps. The Vaseline will stop the water touching the seal, hence increasing that component's life span

When any seal is starting to feel harder than it should, get it replaced.
HTH
Rabbut