It appears as though D-Link no longer supports fun_plug as of firmware version 1.06b03, so the problem is intentional by D-Link.
The info below is a bit verbose, but I spent countless hours trying to get fun_plug to work before spending even more hours searching until I unraveled this mess so I am including the info here for completeness in case it helps anyone else.
D-Link did not initially publish that the fun_plug feature was removed, but it is now starting to appear in some release notes as D-Link is retroactively inserting the information (for instance, as of 2015-09-09 there were no release notes on any D-Link website saying fun_plug support was removed from 1.06b03 (on that date I had checked USA, Canada, UK, and Russian D-Link sites and there was no mention of it, even in forums).
I first saw reference to it 2 days ago in a Russian forum where a poster said "Fun_plug script was removed and no longer to be supported" (at "http://forum.ixbt.com/topic.cgi?id=109:95-56" a poster on "25.09.2015 09:38" says this).
That prompted further searching and I finally found that same exact phrase in the D-Link Canada release notes, yet those release notes were not there before (ftp://ftp.dlink.ca/PRODUCTS/DNS-320L/DNS-320L_REVA_RELEASENOTES_1.06.PDF). This D-Link Canada website shows the release date as 2015-06-30, but the release notes say 2015/07/28 release date, yet I know they were on on the website as of 2015-09-09, so although the dates are very suspect, at least the release notes do mention that fun_plug is no longer supported (http://support.dlink.ca/ProductInfo.aspx?m=DNS-320L).
D-Link USA's website STILL TODAY shows NO MENTION of fun_plug functionality removal (ftp://ftp2.dlink.com/BETA_FIRMWARE/DNS-320L/REVA/DNS-320L_REVA_FIRMWARE_PATCH_NOTES_1.06.B03_EN.PDF).
The D-Link USA website shows the release date as 2015-08-06, but the release notes say 2015/04/01 release date and those release notes make NO MENTION of the removal of the fun_plug functionality, but since the 1.06b03 firmware binary file you download at D-Link USA is identical to the one you download from D-Link Canada, if one has the functionality removed then so does the other (both binaries have an MD5 hash of "0216D3AFF50FF31545F160A0226EDE3F" and so are identical).
D-Link UK's website today shows the latest fw as 1.04b12 which it says was released 2015-05-11, so that makes the UK 1.04 released AFTER everyone else had 1.05 already released, and puts the D-Link UK website 2 versions behind as their most recent firmware available (http://www.dlink.com/uk/en/support/product/dns-320l-sharecenter-2-bay-cloud-storage-enclosure).
D-Link Russia's ftp server today lists 1.05b06 as latest fw and is dated 2015-08-04, but release notes say 2015/07/16 release date (http://ftp.dlink.ru/pub/NAS/DNS-320L/Firmware/). In the past I had noticed Russia publishes firmware earlier than others in some cases, but this is not true for 1.06b03.
Basically the official published dates are all over the place, and can't be trusted.
This is rather frustrating being as the fun_plug functionality was the reason I bought my 4th D-Link NAS, but now I am forced to either use old firmware with known security holes, or new fw 1.06 with no security holes but now no fun_plug.
So if anyone knows differently that fun_plug support is coming back, or if there is a work around, please chime in, otherwise it is dark days ahead for our D-Link NASes.