Create a 24-bit .bmp with the airport name in white on a black background. The long dimension will probably have to be 512 or 1024 if it is to look good up close. The shorter dimension can be whatever power of 2 works. Create a second .bmp (same size) of uniform color - whatever color you want the name to be. Use ADE_GP gp_Signs_White texture as a guide.
Then, with ImageTool or DXTbmp, load the first .bmp as the alpha channel and the second as the main texture. Save as DXT3 for FS9 or DXT 5 for anything else.
Save the DXT texture in ADE_GP Textures folder and the 24-bit alpha .bmp in the Textures_dpy folder. Also, add a new entry to Texture_Def.txt as detailed in the ADE_GP user manual.
Now, open your airport with ADE, create a rectangular ground poly of the necessary size and shape where you want the name displayed. ADE will take you to the ADE_GP editor. Assign your new texture to the ground poly and drag the four vertices so that they surround the name on the ADE_GP display. Click OK.