I solved my problem (not having INDEX in my MetaMask wallet at Chrome extension but on my Android MetaMask wallet). Just in case somebody stumbles over that as well:
- open etherscan.io
- search for INDEX and select Index coop
- copy Contract: (0x0954906da0Bf32d5479e25f46056d22f08464cab)
- go to MetaMask wallet and add Custom Token with this info
Now I got the INDEX listed including the amount!