The mod should show up in the MODS menu regardless of the expansion the player has. Most likely the mod has never downloaded from steam (this is an on-going aggravation with the way the steam system works in tandem with the game's mod downloading system). It may also have downloaded, but then never 'unpacked' itself from the zipped version of the mod that you are actually uploading to the steam workshop when you publish your mod. There are several mods that I am subscribed to that I can never get to download properly and I always have to go to where the mod author is offering a direct download method (one not through steam).
..................................................................................................................................................
The following is also relevant
after a player enables a mod that was meant for G&K or BNW, but they only have Vanilla.
There were numerous changes to the XML when G&K was released. There were additional changes with BNW.
If your mod has an xml file with a BNW-only table or BNW-only column
within a table, this will cause conflicts with G&K and Vanilla. Same is true with G&K but the compatibility issues with G&K-only commands will
for the most part only be between G&K and Vanilla.
So if a player only has Vanilla but your mod has a BNW-only command somewhere in an XML file, this will cause one of two types of results:
- An Incorrect/Missing Reference error will get posted to the Database.log but the game will load everything else.
- The problem column or table will cause the game to discard the entire XML file.
#1 is annoying for the player, usually, because in such cases your mod won't actually do everything you as the mod author say it does.
#2 can have a cascading effect throughout your entire mod.
As an Example: If your mod is a custom civilization and the XML-file that had the unnacceptable command (for Vanilla) was your Unique Building, your civ would not load into the game with the required 2 Uniques. Selection of civilization in the game set-up menu would become 'locked' because when at least one civilization does not have its required minimum two uniques, the scroll-bar for selecting your civ in the game set-up menu will disappear.
[edit] basically saying the same as turingmachine just I took far longer to type it, and was far more wordy about it