virtualbox.org

On my machine Win7 coreinfo -v shows different capabilities on a host and a client even VT-x/AMD-V and Nested Pages are enabled:

On the host:
Intel(R) Core(TM) i7-2630QM CPU @ 2.00GHz
Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
HYPERVISOR - Hypervisor is present
VMX * Supports Intel hardware-assisted virtualization
EPT * Supports Intel extended page tables (SLAT)

On the client:
Intel(R) Core(TM) i7-2630QM CPU @ 2.00GHz
Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
HYPERVISOR - Hypervisor is present
VMX - Supports Intel hardware-assisted virtualization
EPT - Supports Intel extended page tables (SLAT)

Is it anyway to really enable VT-x and SLAT inside a client?

VirtualBox 4.2.6

No. They are hardware specifications which are not simulated/emulated in a vm.

what VT-x/AMD-V and Nested Pages checkboxes mean in this case?

VT-x/AMD-V are the technologies names for the Virtualization part of the CPU
Nested pages is another technology to optimize memory access from a VM as part of the Virtualization process

More details in the user manual here