Its app icon resembles the word "imo" inside of a speech bubble on a white background. If you aren't signed into imo on your computer, you'll need to sign in using your phone number. It's a tab in the upper-left corner of the window. A drop-down menu will appear. This option is in the middle of the drop-down menu. Doing so opens a list of your currently blocked people on the right side of the window. If you've blocked more than one person on imo, find the account that you want to unblock. It's below the person's name. This will immediately unblock the person. Click the person's name to open their profile page, then click Add to Contacts at the top of the chat window.

Summary:
Open imo. Click imo. Click Blocked Users. Find the account that you want to unblock. Click Unblock. Add back the person to your contacts.