Hello,
thanks guys now it works.

The trick is as follows (implemented on PC)
1 - got to http://xdxf.revdanica.com/down2/
2- select your dictionary
3- slect download format Stardict
4- extract it with WinRAR.
5- some dictionaries have 2 links. e.g. stardict-comn_sdict05_eng_arabic-2.4.2.tar.bz2 and stardict-comn_sdict_axm05_eng_arabic-2.4.2.tar.bz2. in this case select the one with _sdict_axm05
6- copy the 3 files .dz, .idx and .ifo to your N900 mstardict directory