Concerning the alsamixer, I assume I have to type "alsamixer sndrpihifiberry" or "alsamixer snd_rpi_hifiberry_dac"? If so, both give me "cannot open mixer: No such file or directory".
No, you should just type alsamixer and note the mixer's name (4th row on the top left menu). For instance, in the screenshot below the mixer is named Headphone.
However, based on the error you're receiving (cannot open mixer) it looks like the device has no hardware volume control (mixer). You may be able to add a software volume control, following the instructions from this page.