Place the cursor where you want the box
Go to the Insert tab
Click Symbol
Select More Symbols
Choose Wingdings or Wingdings 2 as the font
Find and select the checkbox symbol
Click Insert
Close the Symbol window
To make it tickable, insert a Developer checkbox control:
Go to File
Click Options
Select Customize Ribbon
Check Developer
Click OK
Go to the Developer tab
Click Check Box Content Control
Insert the checkbox where needed