You have to be using an original 3DS/3DSXL to use the browser (SpiderHax) exploit. If you're using a New 3DS/New 3DSXL, you must use a save-game hack.
If you are using an original 3DS/3DSXL, make sure to reset your browser, using the settings menu inside the web browser, and selecting "Initialize Save Data", which wipes the browser clean, and removes any data that could cause problems with the exploit running.
Additionally, you must make sure you have correctly copied the Gateway3DS Launcher.dat file to your internal 3DS SD card.