Cats can be difficult to pill. For some cats, adding a little butter to the medication can make it easier to slide down their throats and hide some of the bitter tastes that medications can have.
To start – try making the pill giving experience "fun". Try offering it as a big treat to start. I have one cat on a daily medication that will eat the pill when coated with butter. You can always resort to opening their mouth and forcing them to take it but many times you can start by making it "fun" and some cats will actually eat it.
For another tip on medicating your cat, please read Tip on Making it Easier to Medicate your Cat.