First question though, was magento working properly before you installed the hellomix? were you able to see a front end and an admin backend when you went through the /install process?
My suggestion would be to uninstall magento. make sure you have the latest version 220.127.116.11 found here http://www.magentocommerce.com/download/ Once you have done that. You will then want to make sure you set all of your sites permisions correctly according to your hosting company and magentos start up instructions which can be found here. http://www.magentocommerce.com/wiki/magento_installation_guide Once you have that set up and magento is working properly within your front end and back end. Then you should be able to install the theme simply by hellothemes instructional video.
When installing the theme there is some tricks. Make sure you go to your Admin page. Once there go to the System Tab, you will get a drop down, then go to Cache Management, Once there you want to check the little select all there should be 7 caches selected, then on the right side there is a little action dropdown that has 3 options, select Disable and then hit the big orange submit button. That will disable the caches to make sure you dont have to keep refreshing to get your site set up. Once you have the site installed and products uploaded then you go back and do the same process but you are pressing Enable and submit this time.
When installing the theme dont try and use hello upload. I prefer just to do it manually. Just open the download from hellotheme. ie.(hellomix folder) you should then open the folder entitled "Theme" there should be three files in there called: App, Media, Skin. Take just those three and Compress them using Winrar or any other compression type programs.. Make it a .zip file.
Now go to your FTP client and upload them to the same place you uploaded magento to which should be your main root directory "/". Once that is done you can then go to your admin backend and go to System>Design>Add Design Change go to the dropdown, select HelloMix submit and Voila it should be set to go.
I am not affiliated with nor do I work for hellothemes. I am just trying to give some friendly advice on how to fix your problem. Please make sure you backup all of your current work before trying any of my advice.
Sometimes the hardest problems are the simplest and just take uninstalling and reinstalling to fix. if it does not work then let me know what happened and I will be glad to look further into it with you.