ok. easiest way: connect your n900 to your pc.
after that copy dictionaries in documents folder. after that, disconnect n900 from pc and use xterminal to move it in .stardict/dic
if you don't know the commands:
open xterminal
cd MyDocs/.documents (don't forget "M" and "D" capitals, in linux is different from "m" small unlike from windows)
when you are inside .documents directory type:
mv "name of dictionary folder" /home/user/.stardict/dic

eg: mv stardict-bulg_syn-2.4.2 /home/user/.stardict/dic

(if you know linux terminals you can use the tab on the screen to autocomplete commands, namefiles...)