Livebox network extension with Airport Express

I have an Inventel Livebox which connects wirelessly to my laptops (2 iBooks and a pc) but the signal is weak in certain parts of my home; I bought an Airport Express with the sole intention of placing it strategically in my place so that it can relay the signal from the livebox, however after following all the instructrions it seems that either I'm a total idiot (quite likely) or the Livebox cannot be configured for this.

Any help on this would be welcome - including getting a different relay station than the Express, if you can recommend a model or models that you know will work with the Livebox would be fine! the Wanadoo help desk refuses to comment on whether or not this should work, and are saying it's my problem.

Thanks