[plt-scheme] Slideshow doesn't show some characters

Hi all,

I'm running X under Linux and in the initial slides of the tutorial of
slideshow some characters appear as a box. For examples, on the slide
which shows the keys to press to control slideshow. 
,sym:rightarrow
,sym:leftarrow

and the other occurrences of this symbols in the slide show up as a box.

Can someone please tell me if I need any special fonts or is there any
special dependency I should know about?

Cheers,

-- 

Paulo J. Matos : pocm [_at_] mega . ist . utl . pt
Instituto Superior Tecnico - Lisbon
Computer and Software Eng. - A.I.
 - > http://mega.ist.utl.pt/~pocm
---
        -> God had a deadline...
                So, he wrote it all in Lisp!