If you use Half-Life, you need to update to 1.1.1.0 and if you use Counter-Strike, you need to update to Version 1.5 . You can find these easily on Google and once done, install Beta 1.1 of ESF and you're all set.
(Ofcourse, these updates have to be applied on the original/retail version of Half-Life/Counter-Strike - whichever one you use.)