I'd suggest you copy my batch file earlier in the thread and use it as a starting point for launching MAME. Here's the reason: If you just start mame64 and run the TI 99/4A, it doesn't have anything attached to its PEB (no disk, no memory expansion, nothing) - so the option to mount a disk into a disk drive is not presented to you.
If you use the batch file I have above, it plugs in a 32K memory expansion, a TI disk controller card, and a speech synthesizer (or in the case of the 99/8, it attaches the hex bus disk controller). Then, you'll get the option to mount something into a disk.
If you can't get it to work that way, I can go one step further and zip up the entire contents of my C:\mame0198 directory, which will have everything already configured the way I use it, along with the ti99.bat file that gives you the menu. All the carts will be in there, some of the disk images I have and maybe even a cassette - I can't remember exactly what all is in there.