What browser are you using? The page/view encoding for Denshi Jisho needs to be set to Unicode (UTF-8). With Internet Explorer you may also want to deselect the "Auto-select" feature, which will automatically detect your system's default, usually ISO 8859-1.
Although I'm positive this is an encoding issue, your Japanese language font (both webpage and plain text fonts) also needs to be set to something like Arial or whatever font your system has installed that can support it. In Internet Explorer, go to Tools > Internet Options > Fonts (Appearances), then select your language and fonts. With Firefox it should be Tools > Content > Advanced (Fonts & Colors), then select your language, fonts, and encoding.
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