Firstly, right click anywhere in the blank space within any tab of the main Excel nav bar and select customise ribbon.
This will open Excel options, under customise ribbon, check the developer checkbox.
Doing so will give a new tab on the main Excel nav bar called Developer.
Now that is enabled, to add a check box, go to insert under the developer tab/form controls and select checkbox. Once selected, click anywhere on your worksheet to add it to the workspace.
This will have added a check box to your worksheet and can be manipulated however you wish.