Right-click on the 3dfx icon in your system tray, and select the top option "Voodoo5 5500 AGP - 3dfx Hub". Select the "3dfx Anti-Aliasing" tab (normally the default selected tab), and then click the button labelled "OpenGL/Glide". You will see a drop-down box called "Anti-Aliasing", which has a choice of options. Select either "2-Sample Anti-Aliasing" (decent anti-aliasing, light impact on performance), or "4-Sample Anti-Aliasing" (excellent anti-aliasing, heavy impact on performance). Select an "AA Demo Key", if you like, so you can switch the FSAA on and off, to see the effect. Click OK.
Now run ePSXe with my plugin, but you may want to drop the resolution down a notch or two if you are using 4xFSAA (effective fill-rate is reduced to 1/4 in this mode, but the FSAA effect makes up for it).
Toggle the AA Demo Key to see the difference between FSAA and no FSAA.