You can only load 3rd party VST Plugins by using the following Studio One versions:
Studio One Producer 
Studio One Professional

Please refer to the articles below for more information:
Studio One Comparison Chart
How do I add a 3rd party VST to Studio One?

For Mac
1) Launch Studio One and navigate to Studio One -> Preferences -> Locations -> VST Plug-ins


2) Make sure the correct location is added:
VST : Library/Audio/Plug-ins/VST/
Audio Unites : /Library/Audio/Plug-ins/Components/


3) Please tick the Scan at startup option, make sure Studio One scans your plug-ins at the program startup.

4) Now refer to this video here to record your guitar track with input monitoring. (In this video, Studio One's built-in amp simulator is used. Just replace it with BIAS FX 2 and you're good to go)


For Windows
1) Launch Studio One and navigate to Studio One -> Options -> Locations -> VST Plug-ins
2) Make sure the correct location is added. The location of plug-in files are depend your setting during installation.

The default locations are as below:
32bits - C:\Program Files (x86)\VstPlugins
64bits - C:\Program Files\VstPlugins


3) Please tick the Scan at startup option, make sure Studio One scans your plug-ins at the program startup.
4) Now refer to this video here to record your guitar track with input monitoring. (In this video, Studio One's built-in amp simulator is used. Just replace it with BIAS FX and you're good to go)

For Mac & Windows

Reset Blacklist

Navigate to the "Studio One" drop down menu, select "Options under the "Locations" tab-> click "Reset Blacklist".