i had the same problem @19 months ..it took three days of...
read them a story on the potty give them a drink on the potty weeing on the potty myself (saying Clever mummy!! OOOOh! mummy did a wee wee on the potty!! Clever. Mummy gets a chocolate button...!) playing cbeebies/iplayer/ youtube on a laptop in front of the potty whilst they sit, stopping the play when they stand up.
drinks for potty-sitting is great as it increases the amount of wee too (though to begin with expect this to go on the floor, even a tiny drip in the potty should be rewarded so long as they know they did it)
save the sweets for when they wee on it (as bigchris says)
i might add my daughter has been exactly the same with every new thing (even toys) until she saw another kidusing a sit-n-ride she wouldn't sit on hers. Thus three days to get her to sit on the potty was no suprise.
do you have a friend whose kid could wee on the potty to show them how its done?
potty training takes Lots of patience. remember: It is not obvious to them what you want, they don't know what the potty is for.
I had a similar problem with DD she sat down and immediately jumped up like she was sitting on a ring of fire - I tried books/tv/bribes but patience was the answer in the end.
I got teddy to sit and count how many seconds she could sit on by clapping - we started at 1 sec working up to 30+ took a couple of weeks though !! then i could move onto the next stage of actually getting something in the potty !!