If you drag an NPK into the Files window of a router you are connected to with Winbox, it should show up as a file in the file list. Once you reboot the router, the NPK will be installed, at which point it will no longer show up in Files if you look there. But if you look in System > Packages after reboot, this represents the list of *installed* packages, and you *should* now see "dude" (or whatever NPK you uploaded) in that list. Once a package has been successfully installed, only then will the new commands that the package provides (such as "/dude") be available to you.
If you don't see the NPK you uploaded showing in System > Packages after reboot, then that means the install was unsuccessful for some reason. You can also confirm by both 1) looking at Files again (if the NPK file is still there after reboot then it did NOT get installed), and 2) looking at Logs (if package installation was unsuccessful there will be a log entry to that effect).
If package installation was unsuccessful, it is usually for one of two reasons: 1) Architecture mismatch, 2) Version mismatch. Both the package architecture and the version have to match the copy of RouterOS that you are trying to install that package on. You can't install a package meant for RouterOS 6.42.4 on top of RouterOS 6.42.3...the version # has to match exactly. And you can't install a package that was built to run on RouterOS for Intel CPUs on top of RouterOS that's running on, say, an ARM CPU (for hopefully obvious reasons). That's why every NPK file has the version # and architecture name built right into the file name itself.
You can see what architecture your Routerboard model's CPU is by going to System > Resources and looking at "Architecture name". Below that, you should also see "Board name".
Please note that "mipsbe" and "mipsle" and "mmips" are all separate and incompatible architectures (e.g. note the extra M in "mmips")! And some newer revisions of a Routerboard model change from one architecture to another! So for example, the original RB750G model was mipsbe (MIPS CPU instruction set, single core, "be" == "big endian"). The variant of the "RB750G" line that is currently in production has a marketing name of "hEX", but internally it calls itself the "RB750Gr3" (750G revision 3). The r3 uses a MMIPS CPU (MMIPS == multi-core MIPS; this is a dual-core CPU!).
Note that "mipsbe" architecture does not have a Dude package available for download. That class of hardware does not have enough resources to run Dude server (vast majority of them have CPUs that would be underpowered to run it, plus generally not enough RAM or internal storage, either), thus MikroTik does not make Dude available for that architecture.
You said you have an RB750, but it is not clear what you mean by that. Do you have an actual device that literally calls itself RB750, or did you mean RB750G (G is for Gigabit)? And if the G, do you have the latest r3 variant or an older one (check System > Resources > Board name)? If you did not mean the G, then you will not be able to run Dude, for every revision of the non-G 750 is mipsbe.
Hope this helps,