Buy something better,
This review is from: Silverline 633860 Mini Grease Gun 120cc (Automotive)
The product claims to take either cartridges or 'loose' grease. The cartridges are hard to find (the standard size is 400cc) and they are used very quickly. If you try and fill with grease then the grease will just migrate past the plunger.
I did get this to work, and it does fit my nipples and otherwise work well, but you have to use the cartridges and this size just is not very common. It is also too small; you can use one up in just a minute or two.
Buy a bigger grease gun, one that takes standard cartridges or works from a tub. Buy one that has a flexible feed hose. Buy one that you can see demonstrated (on youtube or at a local shop - and then buy there!).