You can work it with a rolling pin too Elin. It kind of needs worked until it softens out and by then you should be able to see fibres. If you are careful you can sort of peel them bit by bit.
These are one of the fibres that folks used to work with their mouths, but unlike flax it'll not break your mouth out in sores.
Sounds kind of gross to folks nowadays but it worked for millenia. Once you do get the tendon soft, if you can keep it in a pouch of something greasy, it'll be easier to use. Traditionally it was a bag made from birdskin that was used. Pluck the bird, sook off the fat inside the skin (carefully, you don't want to pull the feathers through the skin, and work it until its soft and flexible. Then smoke it. Turn it inside out and the grease in the feathers will keep the sinew in good condition. Don't let it get damp though, just keep it cold.
Alternatively you could use Willowbark's recipe and instructions
http://www.bushcraftuk.com/forum/showthread.php?t=9971
Artificial sinew makes life easy
but it's good to know how to do without, iimmc.
cheers,
M