Thanks to apicmd it has build in help so once installed you can type:
!apicmd -h power
And it will list all the options and what they do.
But I will give some examples of use.
First off i would recommend always using the command in a ability that is set to a token action, this prevents any mistakes as much like many other auto damage scripts you need to pass it the id of the selected token so it knows who is making the power.
So here is the most basic example:
!power @{selected|token_id} "An attack that does nothing."
These are the two required options the first is the token id that is linked to the character that is performing the power, and the second is the name of the power.
Ok so as an example I add an at-will from my wizard I am playing at the moment.
First off I set the attributes for my wizard and my dummy monster:

Next I make the ability, for this example i will be making Illusory Ambush:

And this is what it looks like as a macro (please note the "Show as Token Action" is ticked):

As you can see i have added a bit of flavour text but you dont have to, then I have the power command:
/me shoots a bolt of dazzling multicoloured light from his staff.
!power @{selected|token_id} "Illusory Ambush" -t @{target|token_id} -a [[1d20+6]] -d [[1d6+6]] -c [[1d8]] -v Will -y Psychic -h "The target takes a -2 penalty to attack rolls until the end of my next turn"
Its fairly simple but i will break it down:
- @{selected|token_id}: this is one of the two required parameters, it is the id of the token that represents my wizard.
- "Illusory Ambush": this is the second of the two required parameters, it is the name of the power.
- -t @{target|token_id}: -t is short for --target, it sets the id of the target token, in most cases you should use @{target|token_id} for this.
- -a [[1d20+6]]: -a is short for --attack, this is the attack roll and must be an in-line roll.
- -d [[1d6+6]]:-d is short of --damage, this is the damage roll and must be an in-line roll.
- -c [[1d8]]:-c is short for --critdmg: this is the extra damage rolled when there is a crit, it must be an in-line roll.
- -v Will: -v is short for --vs, this is what defence the attack is made against.
- -y Psychic:-y is short for type, this is the type of damage being dealt.
- -h "the target .....": -h is short for --hit, this is extra text to show only on hit.
Then to use the power all you have to do is click on the token representing the wizard:

Click the power at the top left and select the target:

Once you have selected the target (in this case a dragon) if you hit damage will be delt:

A text output will be given for a hit:

A miss:

Or a Critical Hit:

Here is a full list of commands taken from the code:
['-t', '--target [token id]', 'use @{target|token_id} with this'],
['-a', '--attack [dice roll]', 'The attack dice roll'],
['-d', '--damage [dice roll]', 'The damage dice roll'],
['-c', '--critdmg [dice roll]', 'The critical hit damage dice roll'],
['-o','--tohit [value]', 'What you are trying to hit'],
['-r','--toCrit [value]', 'Crit Range'],
['-h','--hit [string]', 'text to show on hit'],
['-e','--effect [string]', 'effect text to show'],
['-m','--miss [string]', 'miss text to show'],
['-l','--missHalf', 'Do half damage on Miss'],
['-y','--type [string]', 'Type of Damage Being delt'],
['-v','--vs [AC,Fort,Ref,Will]', '.....']
The only odd one is toCrit this expects the lowest number you crit on (default is 20), so if you put "-o 18" you would crit on a 18, 19, or 20.
That should be about it, just ask if you have any questions.