I'm not able to see the Actions tab in the repository.

Update August 2020, 10 months later, see:

GitHub Actions improvements for fork and pull request workflows

^{written by Chris Patterson}

New settings for private repository forks

[...] To enable these users to run workflows on fork pull requests, we’ve introduced three new settings at the enterprise, organization, and repository level for private repositories only.

Improvements for public repository forks

[...] we’ve added a new pull_request_target event, which behaves in an almost identical way to the pull_request event with the same set of filters and payload.

However, instead of running against the workflow and code from the merge commit, the event runs against the workflow and code from the base of the pull request.
This means the workflow is running from a trusted source and is given access to a read/write token as well as secrets enabling the maintainer to safely comment on or label a pull request.
This event can be used in combination with the private repository settings as well.

No further actions are required, as long you or your organization is signed into beta. You should see Actions running on next push to fork, or however workflows are set. It's also often suggested to commit changes to workflow file(s) in your fork (editing those coming from upstream or adding your own).

If it takes too long, you can always report it on Actions forum; i've seen a few threads about exactly same problem in last weeks, with GitHub staff fixing it manually.