I keep receiving this message in my Notices:

ATLAS@home: Notice from server
VirtualBox is not installed
Wednesday, January 04, 2017 'AMt' 02:35:44 AM

I am using a MacBook Pro running El Capitan 10.11.6

I installed VirtualBox hours ago and have restarted BOINC and my machine as well and this error message still persists.

Can you help, please?

Which VirtualBox version did you install, and does ATLAS say it is compatible with their project? Check that link for all the requirements for running work in VirtualBox for ATLAS.

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Did you enabled virtualization in your BIOS?

Since you have i7-3720, I've often seen with Intel 2nd, 3rd, 4th generations it's disabled by default.

On a Mac you cannot enable / disable virtualization (and there is no BIOS), it is always enabled.

I have

03-Jan-2017 22:06:52 [---] Starting BOINC client version 7.6.33 for x86_64-apple-darwin

03-Jan-2017 22:09:06 [---] Processor: 8 GenuineIntel Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz [x86 Family 6 Model 30 Stepping 5]
03-Jan-2017 22:09:06 [---] Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clfsh ds acpi mmx fxsr sse sse2 ss htt tm pbe pni dtes64 mon dscpl vmx smx est tm2 ssse3 cx16 tpr pdcm sse4_1 sse4_2 popcnt
03-Jan-2017 22:09:06 [---] OS: Mac OS X 10.12.2 (Darwin 16.3.0)
03-Jan-2017 22:09:06 [---] VirtualBox version: 5.0.30r112061

03-Jan-2017 22:09:11 [ATLAS@home] URL http://atlasathome.cern.ch/; Computer ID 57; resource share 300

and my Atlas is running fine

04-Jan-2017 00:41:56 [ATLAS@home] Computation for task slVLDm7TchpnDDn7oo6G73TpABFKDmABFKDmqfMKDmABFKDm3iY9am_0 finished
04-Jan-2017 00:41:58 [ATLAS@home] Started upload of slVLDm7TchpnDDn7oo6G73TpABFKDmABFKDmqfMKDmABFKDm3iY9am_0_r164624136_ATLAS_result
04-Jan-2017 00:42:00 [ATLAS@home] Finished upload of slVLDm7TchpnDDn7oo6G73TpABFKDmABFKDmqfMKDmABFKDm3iY9am_0_r164624136_ATLAS_result

So I have Sierra but not long ago I had El Capitan and Atlas was working too.

VirtualBox is not installed
..
I installed VirtualBox hours ago and have restarted BOINC and my machine as well and this error message still persists.

Can you help, please?

.. installed hours ago ..?

Could it be that you had a work request to ATLAS in the time VirtualBox wasn't installed.

Maybe the reason for next flag set in your client_state.xml:

<p_vm_extensions_disabled>1</p_vm_extensions_disabled>

Please follow the checklist Jord already mentioned Checklist Version 2 for Atlas@Home on your PC

Another possibility is that the message is not coming from ATLAS, but from LHC@home.

The message is sent (why and how to suppress is under investigation), when SixTrack tasks are requested, but the LHC@home server don't has tasks available.