I've added several new resources in my mod, and I've managed to make them displayed in the city info screen (in the top right panel) modifying the GameFont.tga file.
Now, what I need to do is to display this button, when creating a message text from the SDK containing a BonusInfo.GetButton(). I can show buttons for the existing resources (rice, wheat, etc), but it shows a pink box for the resources I've added.
Any suggestions? Thanks!!

Not exactly sure what you mean but in CIV4ArtDefines_Bonus.xml there's a button-tag for each resource. Here you can define your button-art.

I did it, and in fact the button is correctly showed in the upper-left panel in the city info screen. But when I try to compose a message by SDK (for example, a text like GIVES +2 HAPPINESS FROM corn_symbol), it works for every resource but NOT for the ones I've added.

Did you try modifying GameFont_75.tga in addition to GameFont.tga?

Cheers,
ripple01