There is no insert button on a mac. If using a program that requires the insert button, you can try changing settings. If you are just typing over text, highlight the text and start typing to write over it.
What do you mean by insert??? If you mean on your keyboard you need to be aware that if you were used to a PC keyboard, MAC keyboards are different. They will not have the same buttons as a PC version. For example there is no control key on a MAC.
You will not be able to find the insert button on a keyboard that is shipped from Apple. The insert button was removed from the Macintosh keyboard. The command is usually located in a menu located on the menu bar at the top of the screen. Sometimes it is buried within another menu such as Format.