1. Make shure that you uploading theme .zip, not full zip package from themeforest
2. Connect to your FTP server and check the CHMOD of the folder: your_eshop/themes and set it to 777. Try to import.
3. Increase max upload size in php configuration
4. If non of this help then using ftp client copy theme_x_x.zip(example theme_3_8.zip) to your_eshop/themes then instead uploading theme_x_x.zip in installer select in from ftp