Highly unlikely that it will grow to a tree but if you want to try simply plant in a pot with compost and ignore for several years.

Still in its husk, it may not be ripe yet. Normally you have to wait until they naturally fall out of their husks, plant them right away, and wait until spring--maybe spring of 2013--for them to sprout. If you get a tree, remember that black walnut trees release chemicals from their roots that kill many other plants (google "phytotoxic"), so you will have to be careful about placement.

thanks folks!
i dont think there is any more room in my garden for trees atm but i was thinking about planting it in a pot so cheers for the advice.
xx

Bad news for those of us in the USA who have Black Walnut trees on our property. Should you have a need to remove such a tree due to storm damage or other reason, forget all about contracting with a tree removal service they will not touch it. The reason is thousand canker disease which is being vectored by the black walnut twig beetle. It first appeared in the western states and now is hitting the east coast black walnut trees very hard. The tree removers find it impossible to clean the fungus spore off of their tools and there are many communities passing ordinances that prevent the removal of any part of that tree from ones property. Example, a black walnut tree came down upon the roof of a homeowners garage during a recent storm in eastern Pennsylvania. The property owner has called various services to remove it but due to restrictive ordinances recently passed none will come. I assume that sooner or later he will get help but it's cost will be very high.
Claire, you will never know if you don't try- Go ahead and plant that black walnut if you haven't done so already!