Hardware Tycoon 0.2.8 and later allow for loading mods in 2 ways:
Hardware Tycoon Mod Tool (.htmod)Edit
If you have saved your mod to a .htmod or downloaded one, simply drag and drop the file anywhere in Hardware Tycoon. You can also go into options and click 'Enable, disable, load or delete mods' at the bottom, then 'Load a Mod' and select the .htmod you want 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 for now.
Remember to create a backup of your game before attempting this!
Package.nw is an archive which contains all of HT's resources. This method allows for much greater customization. You can change sprites, texts, 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 the one you've downloaded. If it's some file that's not package.nw, you have to open the package.nw in your game's folder as an archive using 7zip, WinRar etc. and drop the file in there, then press 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 0.2.8 or later you can use both methods at once, simply load .htmods into your previously modified game.