Thanks Ricardo, finally got it working!
mamemocjk-him-scim-anthy is a good solution to my japanese dictionary woes, but not a perfect one. I have a couple of remarks/questions:
1. GUI of anthy looks a bit "off" on my N900, some buttons overlap, etc. It seems that the GUI has been designed for some other resolution, and hastily ported to N900. Can it be changed? I might have a go since I've done some GTK programming, but only on Windows, none on N900. Any opinions, where to start, how difficult would it be, etc?
2. Is there a way to use anthy without having to close the HW keyboard?

Now, related to the reason as to why I need japanese input (learning japanese!), I would appreciate very much if somebody could advise me about this:
1. Which dictionary would you recommend for Japanese? I am using MStarDict, with JEDict, I'm but not entirely happy with the verbosity of entries and with the size of vocabulary.
2. Is there some software for searching kanjis, based on radicals (e.g. like at jisho.org), or by a direct input?

Many thanks!