Connect one end of your USB cable to your MIDI keyboard, and the other to your computer.
Click the shortcut on your desktop or in your Start menu to launch your ReWire host.
Click the shortcut on your desktop or in your Start menu to launch Reason 4.
Open a new project in your ReWire host application and configure Reason 4 as a ReWire slave. Then, load tracks to your ReWire host to connect to your Reason 4 output tracks. The method for doing so will depend on which ReWire host you are using. Check your program's help section or user guide if you are unsure.
Click "File" and then "New" to open a new project in Reason. You can now add mixers, effects and instruments to your Reason rack by right-clicking on an empty area above the sequencer lane and selecting the virtual devices you want to use. When you select a device, it will appear in your rack and be allocated a track in Reason's sequencer.
Route each Reason instrument to a separate track in your host program. You should be able to do this by choosing each Reason instrument from a drop-down menu on each track you've loaded in your host program. The exact method will vary depending on which sequencer you're using as a host. Sequencers you could use include Cubase, FL Studio and Ableton Live. These can all act as a ReWire host and support VSTi plug-ins.
Load another track in your host program and connect it to an instance of Toxic Biohazard. In most programs you'll be able to do this by right-clicking on an empty area of your host's sequencer window and selecting the option to add another MIDI track. You can then load Toxic Biohazard from your VSTi plug-in collection, which should be accessible from your host program's toolbar and select it from the drop-down menu on the track you've just created.
Build your tune in your ReWire host on your MIDI keyboard using Reason 4 instruments and Toxic Biohazard.