From my perspective, there are several things you can do:
1) Check whether the corresponding snap-in is loaded, indeed, or it's just not list among others.
2) I don't know for sure, but there has to be some kind of C# log, where you can check and see whether there are any issues with snap-in adding.
3) You can start from the different angle, executing C# commands from the PS. This way, you will be able to use both Veeam PSsnapin and C# commands.
Anyway, it will be appreciated if you provide a little bit more details regarding the task you're trying to achieve.