SO jealous that you're in Salvador - I was there in Feb and July last year and am looking out at the grey UK weather wishing I was back in Bahia.
The place most tourists go is to the Academia de Mestre Bimba in the Pelorinho. Run by the legendary Mestre Bamba they do regular rodas (when capoeira is plaed raher than trained) and you can pay to go watch). Bamba and his students are great so it's worth a go, but training with them is pretty pricey (cos tourists will pay...)
There are usually random rodas going on around the main Pelorinho squares and usually one down at the mercado modelo, but these vary in quality.
If you're anywhere in the Praia do Flamingo/Abaete area of Salvador then get along to the Criancas Raizes do Abaete (near the Lagao do Abaete). There are kids classes there most mornings and afternoons, and adults classes and rodas friday nights and saturdays... i think...
Ask around in Abaete - Professor Rupi / Prof Pesado / Prof Cui - they're all good guys, and if you go to visit or train with them, please make a donation to the Projeto and tell them Estrela from Oxford sent you.
Enjoy. capoeira in brazil with brazilians is where it's at!
(btw: am a trustee for the charity that supports the Projeto in Abaete - see www.abolicaotrust.org)