imagine you'd be building the PC in real hardware:
You'd have to open the case and connect the 5,25" DS/DD drive, then configure the BIOS to use it.
Same in MAME: Start the emulation, switch to partial keyboard emulation, TAB into the menu, then go to "slot devices" and change the emulated drive to 525dd.
After resetting the emulated machine, configure your BIOS to match the virtual disk drive you just installed, and your 360K disk images will be loaded correctly.
On a physical AT machine, the 5.25" HD (80 track) drive could also read, write and boot from DD (40 track) media, this isn't supported in MAME yet.