Go to the "File" menu and select "Create game from ISO". Select Source and Destination: Source: Find your PS2 game ISO on your computer. Destination: Select the root of your FAT32 USB drive.
The most reputable locations for downloading the v3.00 beta are established homebrew hubs like Legacy Downloads: For those preferring the stable classic version, Internet Archive often hosts mirrors of USBUtil v2.0 and v2.2. Security Note: usbutil 3.0 download