There are several ways to add a new channel in FL Studio's pattern window. These can be samples, loops, FL Studio Generators like FL Slicer, FL Keys, etc or even thrid party VST instruments.
For the sake of this tutorial lets split the lesson depending on what you are trying to add. Well split it by samples and instruments.
This would be any file that is in MP3, WAV or OGG format. These generally repesent one shots (kicks, snares, hats), loops, chopped phrases or pre-recorded material, or a full recorded music phrase or vocal.
To insert a one shot, sample or loop to be used in the step sequencer or piano roll, simply drag the file onto the pattern window and it will insert it into a channel.
If you want to insert a longer phrase, say an acapella you are trying to remix, it's better to drag it into the playlist clip area where you find it easier to adjust. This method will also make a channel for this clip automatically.
When dragging the file, you can drag it directly from a Windows explorer windoe or the built in FL Studio browser. You can also manually add a channel via Channels>Add One, but then you are forced to browse for the file which can be more inconvenient than just dragging it in straight away.
Instruments include VST and DX instruments as well as some of the built-in FL studio generators like SimSynth, FL Keys, etc.
if the instrument is a built-in generator, it may be on the default or favorite list when you select Channels>Add One. Below the More... menu item you will see the default list of items. If your instrument is there, select it and a new channel will be added for it.
If the item is not there, you need to slect More... which brings up a more complete list. Notice that the favorite items appear with a small F in the box next to them. By clicking on this box you can toggle if that instrument appears on your favorites list in the future.
If you still do not see the item listed, you may have installed the VST but FL Studio is not aware of it. Select Fast Scan to have FL Studio look for any new VST plug-ins installed. Once the process is complete, all the new plug-ins will be listed in red to help you spot them quickly.
Find the item you want to add by scrolling through the list and double click to add it.
You have learned how to add sounds and instruments to the pattern window in FL Studio. You also learned about the different types of sounds you may nbeed and the best way to add them.
Click on How to add plugins to FL Studio to learn about that in more detail.