Hi there! The floating is by default a hold input when you're in the air, with Z or C on keyboard (same as jump). You can change the settings on the options menu on the main title screen, so that you just have to tap the button to activate/deactivate float if you'd like.
You can also flip the inputs so that jump and float are on X/V, and slide/groundpound are on Z/C on that same menu. If you're still running into issues, I'd try disabling the Jump Buffers option on the menu if it's on - that option prevents float from activating if you're holding a down input. Hope that helps!