How can I paste commands on Minecraft Command Blocks?

All you need to do is use ⌘+C after highlighting the command you want to use to copy it, and then go into the command block and use ⌘+V to paste it.

This actually depends on what version you're using. Since there's no right-click function in the command block menu, you have to use keyboard shortcuts; however, different versions use different shortcuts. Minecraft 1.7.10 and earlier were all made using the shortcuts for Windows and other systems, so if you're using one of those older versions you have to use Ctrl+V to paste. This gets confusing, because other applications still use ⌘ on Mac. However, if you're using any version of 1.8 or later, you can use ⌘ + V, like the other applications on Mac do.

TL;DR ⌘ + V if you're using 1.8 or higher, Ctrl+V if you're on 1.7.10 or lower