That's only enabled when using the "Use TXI text for all TGA". You can either type, paste, or load the TXI text into the window. In the default state ("Use TXI file for each TGA"), it will automatically load any TXI that is named the same as its parent TGA and in the same directory. Typically you'd normally use the default mode, unless you are batch processing a bunch of textures that all use the same TXI semantics.