Why do I have 0 voting power even though I have $ARB?

Here's a full explanation of how voting and proposals work.

Keep in mind that there are 2 types of proposals:
1 - Proposals are first submitted to the ArbitrumDAO governance forum for community discussion and debate. These forum submissions are usually accompanied by a Snapshot poll that gauges the community's interest in the proposal. For this, you just need to go to Tally and delegate the voting power to yourself (before any proposal goes live, otherwise you still won't be able to vote).

2 - If the proposal passes the temperature check, it will move on to an on-chain vote facilitated by Tally. For this phase, you'll need to delegate your voting power. Either to an entity that is already a delegate or to yourself.

Basically there's two ways to participate in ‌governance: either you do it yourself by voting on every proposal (snapshot and tally) or through the entity to which you delegated your voting power. In this last case, they only need to vote on Tally because that's the final proposal before a decision is executed or not.

We would advise to take a look at this: https://docs.arbitrum.foundation/how-tos/vote-dao-proposals#proposals-in-the-temperature-check-stage