This system would be completely impossible without the remarkable work of Dr. Jim Breen and his contributors to the KANJIDIC and EDICT datasets which form the backbone of KanjiDB. It would also be impossible without the generous licence for the use of that data provided by the Electronic Dictionary Research Group of Monash University.
At present, no mechanism has been implemented for easy entry of kanji characters. Unless you have a Japanese IME running on your computer in some capacity, your best bet is to copy and paste characters from some source, possibly the kanji pages or kanji search.
You should also be aware that the database only searches for one romanization per kana. In the case of kana such as は (ha, but pronounced wa when used to denote the topic marker particle), or ヘ (he, but pronouced e when used to denote the destination particle), the more general pronunciation is the one the romanization engine will render. For example, the result 今日は - konnichi wa (hello, good day) will not turn up on a search for the string "konnichi wa" or "konnichiwa" because the romanization engine does not presently recognize the wa syllable as corresponding to the は kana (it would instead match to わ which is always pronounced wa). Searching for "konnichi ha" or "konnichiha" will turn up the correct result. Of course, searching for the string "こんにちは" takes the romanization engine out of the loop and will return the desired result without having to accomodate the limitations of the engine. Future upgrades to the engine may be better able to handle kana with somewhat ambiguous readings.
There is, naturally, an exception. The kana を is technically pronounced wo but this pronuciation is obsolete (except occasionaly when it is used in names, which often have archaic spelling rules). It is only ever used, ignoring the name exception, to denote the direct object particle (pronounced o) so the romanization of the kana is not ambigious and a search for "o" will turn it up (and a search for "wo" will not). For example, the result 円を描く - en o gaku (to draw a circle) will turn up on a search for the string "en o gaku" or "enogaku".