First you need to click on the kitchen sink button. This will expand a second row of formatting options where you should see the Special Characters button.
Clicking on special characters button will bring up a pop-up window showing some commonly used special characters. It has currency symbols, trademark, copyright, registered signs, mathematical symbols and signs.
Now you need to just click on the symbol or character you want to use, and it will be added in your post.
Adding Special Characters Using Unicode HTML Entities
Unlike Visual Editor, the text editor in WordPress does not have a special characters button. This means you will have to write the HTML entity code to add special characters.
Each character has an HTML entity number assigned to it. For example, 1 has HTML entity number
1. Same goes for just about any character in almost any language. However, remembering these HTML entity numbers is not an easy task.
To solve this problem, most commonly used special characters have HTML entity names. These names are easy to remember due to their similarity to the real word used for these characters. For example, copyright sign has HTML entity name copy, trademark uses trade, and registered symbol uses reg as HTML entity name.
To add these signs you need to append them with an ampersand sign
& and end with a semicolon. See the examples below:
© = ©
™ = ™
® = ®
€ = €
To find out the HTML entity for any character or symbol, you can refer to the character entity reference chart on W3.org website.