In order to create new cards, we will need access to your Trello account.

[Tom Coady]

Using the gmail plugin authorisation is sought but never granted:

Would you like to give this add-on access to your account?
This add-on would like to show additional information in Gmail, but it needs approval to access your Google account.

Nothing happens when I click authorise account access.

I have 2 gmail and 2 trello account in case that is relevant. 

[Tom Coady]

OK I finally figured out what was going wrong using the Safari browser, it automatically blocks all pop-ups, or at least is set to do so for me, and I failed to notice the warning about this in the status bar. Now I was able to grant permission to access Trello the pop-up fails to do much or to close itself:

[Tom Coady]

Sorry, one more update. After receiving a mail confirming the receipt of permission request the power-up / add-on is now working nicely on my gmail :)