Sometimes the mouse inside a VM is unusable - clicking anything is next to impossible ....
This can have different reasons.
If you run a Windows 7 host and have mouse-problems in all VMs make sure that the
"Make it easier to read what's on your screen" settings uses a value of 100 %.
Other settings may result in a strange mouse behaviour.
If you get an unusable mouse inside a Linux-VM make sure the guestOS parameter is set to Linux.
A popular example for this is using a Linux LiveCD inside a VM created for a Windows guest.
As long as this VM uses for example Windows XP as guestOS the mouse is unusable as its behaviour is almost random. If you change that to Linux the mouse is usable again.