There seems to be something weird going on with the Beta Tester plugin, it isn't finding the 3.5 betas
This is a bug and I'm looking into it.
Similarly, when I use it on 3.5b1, it's not finding 3.5b2, only the nightlies.
We don't support just installing Betas at the moment.
The Beta Tester plugin just helps you switch from a release install to a development stream for nightly builds - we actually want people upgrading to each nightly between betas (and RCs) so that we don't get duplicate bug reports etc.