You need to put the file inside a folder, for example modloader/a folder with any name you want, the name of the mod probably/mod content here
If you still don't get it, post the log (modloader/modloader.log) so I can analyze what's going on
I don't have any modloader.log file inside. I have a .data and a modloader.ini file in it. That's all.
Yes the files are in a subfolder that's inside modloader, exactly like "modloader/gta3.img/BMX/bmx.dff&bmx.txd".