I'm running into trouble getting Force IDE v35 to work with Eclipse Mars.1. Everything installs OK and I can create a Force.com project, but my package.xml file keeps reporting this error:
cvc-elt.1: Cannot find the declaration of element 'Package'.
I have uninstalled and reinstalled Eclipse and Force.com IDE plugins. Recreated the project multiple times. Still the same error. I have even replaced the entire computer and installed from scratch, and I still get the same error.
So, I downgraded to Kepler, and installed v35 of the Force IDE plugin and I get the same error. I've also verified the issues "crmprogdev" raised in his answer (1-4) aren't the ones I'm having. I'm starting to think it's JRE related or Force IDE plugin related.
I also downgraded to v34 of the Force.com IDE plugin and I still get the same error.