Don’t worry, this is a common mistake for customers that are new to purchasing themes from ThemeForest.
You’re tying to install the entire zip package you downloaded from ThemeForest, but you need to install the theme found inside. Inside your download package you’ll find another ZIP called “alyeska.zip” along with your documentation, PSD’s and licensing info. — This “alyeska.zip” is the theme you want to install.
This video will walk you through the installation process:
If you have problems from there, make sure open up your documentation; there you’ll find fairly detailed chapter on installation with two videos and common problems people face when installing.