Not sure why it doesn't work for you. I have tested the mapbox custom tileset on both the newest (v3.1.3.113) and a much older version (v3.1.1.91) of Plan-G with no problems whatsoever. While making this tileset over the past month I even changed my "access token" (long code) multiple times and on each new version of the tileset created generated a new map ID (short code). All of which worked in Plan-G.
You could also check the "user.config" file located - C:\Users\
<me>\AppData\Local\TA_Software\Plan-G3
<Magic Number>\
<version number>\user.config
to see if the entries match. (Plan-G shut down) The config file is nothing more than XML and can be opened by notepad or other plain text based editor. I did, at one time, have a problem entering the long code into the "Key:" textbox. The code is so long and the textbox is so small that some stray characters were left over.
From version 3.1.1.91 (uses only the short code, step 6, as shown in manual - Refer to 8.9.9, page 29 ) -
<setting name="MapboxKey1" serializeAs="String">
<value>
username.XXXXXXXX</value>
</setting>
<setting name="MapboxKey2" serializeAs="String">
<value>
username.XXXXXXXX</value>
</setting>
From version 3.1.3.113 (uses both codes, steps 2 & 6) -
<setting name="MapboxKey1" serializeAs="String">
<value>
pk.eyJXXXXXXXXXXXXXXXXXXXXXXXiJjaXRsZzQ5bHEwM3p5XXXXXXXXXXXXXXXXX.IM23hBp2u7DXXXXXXXXXX</value>
</setting>
<setting name="MapboxKey2" serializeAs="String">
<value>
pk.eyJXXXXXXXXXXXXXXXXXXXXXXXiJjaXRsZzQ5bHEwM3p5XXXXXXXXXXXXXXXXX.IM23hBp2u7DXXXXXXXXXX</value>
</setting>
<setting name="MapboxID1" serializeAs="String">
<value>
username.XXXXXXXX</value>
</setting>
<setting name="MapboxID2" serializeAs="String">
<value>
username.XXXXXXXX</value>
Hope this helps.
.