Kelp is an uncountable noun.

Question

How can we use kelp?
I ate a kelp.
I ate two kelps.
(Are the expressions correct? How can we count 'kelp'?))

Answer 1

I don't believe kelp is a countable noun, so your first and second sentences are correct.

Your third and fourth sentences are not correct, though. You'd simply say, "I ate kelp," or "I ate some kelp."

Answer 2

Here's a bit more information. Notice how the word is used in here:
http://en.wikipedia.org/wiki/Kelp

Answer 3

http://esl.about.com/od/grammarforbeginners/a/g_cucount.htm