Modifications (or mods for short) are only supported in desktop releases of Hardware Tycoon. You cannot load a mod in a browser version of the game. If you have installed Hardware Tycoon 0.2.8 or newer, you have two ways of importing mods:
Hardware Tycoon Mod Tool (.htmod)
If you have saved your mod to a .htmod or downloaded one, simply drag and drop the file into Hardware Tycoon anytime after creating a company. You can also go into options and click 'Manage mods' at the bottom, then 'Load a Mod' and select the .htmod you wish to add to your game. You can also disable a mod by clicking the switch next to it or remove it completely by pressing the X.
You cannot modify existing HT content using this method. It only allows adding packages and research.
Remember to create a backup of your game before attempting this!
package.nw is an archive which contains all of the game's resources. This method allows for much greater customization. It's possible change sprites, texts, sounds, some of the game's variables and arrays containing packages, competitors, etc. If you have downloaded a package.nw (for example from ModDB), simply replace the one in your game's folder with that one. If it's a file that's not package.nw, you have to open the package.nw in your game's folder as an archive using i.e. 7zip or WinRar, and drop the file in there, then choose to replace.
Note that if the mod was made for use with a different version of the game that you currently have, replacing your package.nw with it will effectively change the version of your game.
If your desired mod supports version 0.2.8 or later, you can combine both methods - simply load .htmod files into your previously modified game.