Hello Misbah,
I cannot see your particular account due to the information provided, so I tested the process on my own account.
I was able to install and view the free-opencart-1551-black theme successfully on my OpenCart test account. Here are the steps I performed:
Unzip the package on your local machine.
Log into the FTP account.
Navigate to the catalog > view > theme folder and upload the black folder there.
Log into my OpenCart Admin.
Click on System
Click on your store and then Edit
Click on the Store tab.
Select the black option from the Template dropdown.
Click the Save button found in the upper right corner
Be sure to clear your browser cache as well if you still see the old template after re-visiting your site.
Best Regards,
Scott M