A Zip code is what they call a postcode in the USA.
I take it the site your looking at is an American one. If you enter your postcode it may not recognize it and therefore the site your trying to use may not cater for UK residents. What are you trying to find out about your soil? I don't see how a site can determine your soil content by entering a postcode.

I think its the american name for postcode

You are using an American site specifically for American gardens, so it is of no use to you. Like MrsS, I would have thought a postcode much to general to give much information about your own garden soil. The soil in our garden is very different from our next door neighbours.

think we all managed to answer at the same time lol.

Hello Guest, any garden centre will have information & soil testing kits to determine your type. Also have a look at what is growing in & around your area, although not specific it can clue you up as a rough guide.
Scource Books/internet/G.o.y etc.
Now is an excellent time to condition, heart-up ( compost) your soil so it can settle & mature in readiness for next Spring.