Hi,
I had a small problem with 2.1.0rc2 but this was my fault. here are a few things to check.
The new version requires a cache folder called silverstripe-cache which I don't believe is created by default. you could try creating the folder and then run a db/build?flush=1 (add this on the end of the url where SS is installed).
Also check if a theme is defined in the _config file in the mysite folder. for more information search for theme in the forum or documentation this will tell you more than I can put here.
If you have just installed in another folder then make sure there is no .htaccess file in the root folder as this could be causing confusion . alternatively reinstall and start again or temporarily move your old version somewhere else and just install the latest version to try and eliminate if there is a conflict between the two versions just to try and track down what is causing the problem.
I use virtual directories so each ss installation has a unique local url ie local-site1, local-site2 etc rather than accessing via just another folder underneath the main host, this helps keep things seperate although they are still physically stored as folders underneath the main root are accessed differently but this is a personal preference.
let us know how you get on.
HTH
Trevor