1 - Start css64,
2 - Press F10 for menu
3 - Select "1541 Device 8" , press <Enter>
4 - Browse for your d64/t64 rom disk file, press <Enter>
5 - press <F1> to select the disk, <F2> to run it, or <F3> to run it fast. If doesn't work, go to the (previous) disk sreen, move 1 line down, press <F2> or <F3>, etc
To further help, take a look a at the "CCS64 Keyboard Layout.pdf" file