Koko is a native Japanese word meaning 'here' that is almost always written ここ in hiragana. In the past it was written in kanji as 此所 or 此処, both actually Classical Chinese words meaning 'this place', but these are seldom found nowadays.
There is a rather peculiar usage in sketch maps to write koko in katakana as ココ. This is a little akin to writing 'x marks the spot' on a map in English.
This usage can also be found in signs, as in the sign below indicating the location of the advertised premises.
A Google search of the Internet in August 2003 revealed the following distribution of usage:
Results for ココ were too heavily influenced by unrelated words that contained the two letters ココ. To show the general prevalence of koko written in katakana, I carried out searches for the specific forms koko ni, koko ga and koko e.