This happened to me too when I disabled mouse capture. The mouse cursor outside of virtualbox became stuck in one place (because I disabled it) and the new mouse pointer in virtual box becomes invisible. The best thing to fix this problem is to actually use an external device, such as a pc contoller with analog sticks. So, plug in your pc controller of whatever external device you want first, then download a free program called JoyToKey or something similar ( I would suggest JoyToKey, it works perfectly). Now, open JoyToKey and map your keyboard keys, like w,a,s,d or up, down, left, right and left and right mouse clicks, including middle mouse roll, and mouse movement axis (what you use to move a mouse, I mapped mine on my controller analog sticks to move pointer around) onto your pc controller. Next, open virtualbox (disable capture). This way, at least now you can move the pointer that was stuck initially in one place with your pc controller. So, if you want to use in Unity3d, open Unity3d now. Move your pointer using your external device, try to preview a game you made, and now you can look around using the mouse! Alternatively, disable hardware acceleration (drag slider to none instead of full) in Display options when inside virtualbox, (however this doesn't work if you want to use Unity3d, because the pointer still disappears when hovering over program)