To check its a problem with bigtrident, can you get freeciv to use both the "trident" and "isotrident" tilesets (which come with freeciv)?
I think the tileset selector in 1.14 beta shows you all the tileset files it could find in the freeciv data directory. If bigtrident is not on the list, then it sounds like you have not unzipped the bigtrident files in the right place.
To recap, the tilespec files like
should go here:
c:\program files\freeciv 1.14 beta\data\
and my other files should go here:
c:\program files\freeciv 1.14 beta\data\bigtrident\
This should all happen properly if you told WinZip to extract the ZIP file to c:\program files\freeciv 1.14 beta\data\ and ticked the box "Use folder names".
Maybe you used c:\program files\freeciv 1.14 beta\ instead, I think I did the first time I tried to use a new tileset.
(Note - I have used c:\program files\freeciv 1.14 beta\ to mean whereever you installed freeciv, if you want to use bigtrident with both 1.14 beta and 1.13, then you will need to install in for both copies of freeciv)
(N.B. - By default recent versions of Windows will hide the filename extensions in explorer, so "bigtrident.tileset" will showup as "bigtrident", and "tiles.png" will showup as just "tiles" - This is controlled by the setting "Hide file extensions for known file types" reached in explorer from the menu Tools, Options)