I'm seeing the same problem trying to update with the latest "Digital Camera RAW Compatibility Update 6.18". Clicking the Update button just greys out for a second then nothing.
I had seen the "answer" related above, but I do not see a /var/folders/zz directory...
I am seeing SoftwareUpdate related errors in the console:
softwareupdated(444) deny system-fsctl 0x682f
Process: softwareupdated [444]
Path: /System/Library/CoreServices/Software Update.app/Contents/Resources/softwareupdated
Load Address: 0x10480f000
Identifier: softwareupdated
Version: ??? (???)
Code Type: x86_64 (Native)
Parent Process: launchd [1]
Date/Time: 2015-12-16 18:47:30.232 -0500
OS Version: Mac OS X 10.11.2 (15C50)
Report Version: 8
Thread 0:
0 libsystem_kernel.dylib 0x00007fff9d21fba6 fsctl + 10
1 Foundation 0x00007fff9c6921f9 -[NSFileManager _URLForReplacingItemAtURL:error:] + 1748
2 Foundation 0x00007fff9c664b7d _NSCreateTemporaryFile_Protected + 201
3 Foundation 0x00007fff9c66445e _NSWriteDataToFileWithExtendedAttributes + 232
4 Foundation 0x00007fff9c6af2be -[NSArray(NSArray) writeToFile:atomically:] + 226
5 SoftwareUpdate 0x00007fff9e5224cb -[SUMetadataCache writeCacheToDisk] + 77
6 SoftwareUpdate 0x00007fff9e547993 -[SUScan _scanMain:] + 1644
7 SoftwareUpdate 0x00007fff9e54a7f0 -[SUScan performSynchronousScan] + 111
8 SoftwareUpdate 0x00007fff9e540951 __276-[SUScanController refreshAvailableProductsInForeground:limitedToChanged:evenIfConfigurationChange d:initiatedByDeviceConnection:limitedToProductKeys:forCurrentConfiguration:distr ibutionEnvironment:distributionEvalutionMetainfo:installedPrinters:preferredLoca lizations:finish:]_block_invoke285 + 772
9 libdispatch.dylib 0x00007fff9c3e2871 _dispatch_call_block_and_release + 12
10 libdispatch.dylib 0x00007fff9c3d733f _dispatch_client_callout + 8
11 libdispatch.dylib 0x00007fff9c3dbf6f _dispatch_queue_drain + 754
12 libdispatch.dylib 0x00007fff9c3e263b _dispatch_queue_invoke + 549
13 libdispatch.dylib 0x00007fff9c3d733f _dispatch_client_callout + 8
14 libdispatch.dylib 0x00007fff9c3db1cf _dispatch_root_queue_drain + 1890
15 libdispatch.dylib 0x00007fff9c3daa34 _dispatch_worker_thread3 + 91
16 libsystem_pthread.dylib 0x00007fff8d55a68f _pthread_wqthread + 1129
17 libsystem_pthread.dylib 0x00007fff8d558365 start_wqthread + 13
Binary Images:
0x7fff8d557000 - 0x7fff8d560ff7 libsystem_pthread.dylib (138.10.4) <327cecd0-b881-3153-8fcc-4fd4818b7f16> /usr/lib/system/libsystem_pthread.dylib
0x7fff9c3d5000 - 0x7fff9c402fff libdispatch.dylib (501.20.1) <324c9189-2af3-3356-847f-6f4ce1c6e901> /usr/lib/system/libdispatch.dylib
0x7fff9c5d8000 - 0x7fff9c92cff7 com.apple.Foundation (6.9 - 1256.1) <66106cc9-9d97-3846-88d8-4bf6bb508296> /System/Library/Frameworks/Foundation.framework/Versions/C/Foundation
0x7fff9d208000 - 0x7fff9d226fff libsystem_kernel.dylib (3248.20.55) <0e688457-4915-36dd-8798-5c2edee3f1a3> /usr/lib/system/libsystem_kernel.dylib
0x7fff9e4f4000 - 0x7fff9e58eff7 com.apple.SoftwareUpdate.framework (6 - 790) <a4c2e56e-4554-3b6a-acac-5a969bc3f0fd> /System/Library/PrivateFrameworks/SoftwareUpdate.framework/Versions/A/SoftwareU pdate