When connecting an Alesis drum kit to your computer, it will send standard MIDI data, and your chosen drum plugin should respond to the force with which your velocity-sensitive pads are striked.
To balance out and mix the level of your kick pad, cymbals, snare, toms, and hi-hat pedal, open your preferred drum plugin and search for a Mixer page. From there, you can adjust the volume of each drum separately.
If you would like to mix your individual drums using the DAW's Mixer instead of the mixer page found in the drum plugin, you can choose different output settings for the plugin when it is selected as your instrument. Typically, you'll be presented with alternatives that look something like the image below.
Upon selecting a multi-output option, your plugin's channel strip should display a Plus icon or similar, enabling you to create aux channels for each of your drums.
Once your aux channels have been created, you will be able to apply additional plugins to them to further shape the sounds of your drums.
Although the information above relates to the majority of popular and modern software, it's important to note that your output options and the functionality of your plugin will entirely depend on which DAW and plugin you are using.
My recommendation is to use the BFD3 Drum Software, which features it's own inbuilt mixer, and the ability to send multiple outputs to your DAW.