Here is a solution for a bonus to the attack roll.
The fields within the attack to add a modifier to the attack roll only accept numbers, so you won't be able to add the Power Orb bonus there.

The only option I can easily find is to use the Global Attack Modifier field:
Name: 'Power Orb', Roll: '0@{Carl|power_orb}'

Read below in the Other notes'ideas/suggestions as well.
Here is a solution to add a bonus to the damage roll for an attack.
The 'Magic Bonus' field only accepts numbers, so you won't be able to add the Power Orb bonus there.
Options:
1. Put the Power Orb attribute call in the damage field for the weapon. If the weapon is normally a 1d6 damage, then change the 'damage' field to be '1d6+0@{Carl|power_orb}[Power Orb]'. You'll see the Power Orb bonus in the tooltip if you hover over the damage roll.

2. Put the Power Orb attribute call in the damage2 field for the weapon, assuming it's not already being used for that weapon. Change the 'damage2' field to be '0@{Carl|power_orb}' and change the 'type' field to be 'Power Orb'. You'll see the Power Orb bonus in the Damage2 field.

3. Put the Power Orb attribute call in a Global Damage Modifier. Name: 'Power Orb', Damage: '0@{Carl|power_orb}', Critical Damage: (blank), Type: 'Power Orb'. This has the nice benefit of only showing up when the value is not 0. You'll see the Power Orb bonus in the Global Damage Modifier field.

Other notes/ideas/suggestions:
You'll also notice that I included a 0 before each of the 'power_orb' calls. This is to prevent error messages from appearing if the 'power_orb' attribute is mistakenly blank using the trick listed here.
If the Power Orb affects all characters, you can also create a character called 'Power Orb' that is referenced instead of keeping the attribute on each character individually. That would be 0@{Power Orb|power_orb}. Or you could have each character's Power Orb status on the Power Orb character: 0@{Power Orb|CarlPowerOrb}. That would allow you to have a token representing the Power Orb character, and link bar1 to the 'power_orb' attribute and you'd be able to change the attribute simply by updating the token's bar1 value.
Though since you have a Pro subscription, you can also just use ChatSetAttr to modify the Power Orb bonus when needed.