You CAN use Japanese characters in your Forum posts, but you'll need to have a language pack installed on your computer.
The language pack will allow you to change the language you want to type in and also the "input mode," so you can insert hiragana, katakana and kanji characters directly into forum posts.
Language packs are free to download, but you will need to get the right one for your computer's operating system. You can search Microsoft Help pages for more details.
From the Rocket Japanese Team
You CAN use Japanese characters in your Forum posts

Sayaka-Matsuura
October 4, 2007

CatPanda
June 12, 2009
Hey I found a nice site that really throughly shows you/teaches you how to type in Japanese using your English keyboard.
そのサイトはすきです。
(I like this site.)
http://www.coscom.co.jp/help3/inputjp/installime.html
Enjoy!
ファーステンベーグ

r_folsom
October 11, 2009
Demonic, I can't get to that site. I keep getting an error page.

(deleted)
October 11, 2009
Hey this is kind of off-topic but I want everyone to check out this site [url=http://dellfreelaptop.tk/:v17n3oc3]http://dellfreelaptop.tk[/url:v17n3oc3] they are giving people a chance to test and keep a free Dell XPS laptop. Pretty cool.

CatPanda
October 18, 2009
[quo]*Quote from * [redacted]\nDemonic, I can't get to that site. I keep getting an error page.[/quo]\n\nI think I fixed it, if not try this: http://tinyurl.com/ylskfs7