You can attack by selecting your creature then Right-Click (on Mac OS ALT + Click), or tripple-click on your creature to pull up the creature's context menu and select "Attack." Alternatively, you may select your creature then press the "A" key to activate the attack.
If you have previously attacked before, you may use the "Quick Attack" or press "Q." This will make the same attack option as the previous time, all you must do is select the target.
Once you press the Attack command, a Declare Attacks Action Window comes up and asks which attack(s) you wish to use. You may use one item, or you may use all items in the list. d20Pro will not restrict your attack options. However, your Judge/GM might depending on the rules you are using.
For this example, Valeros is a dual-weilding fighter using two-weapon fighting feat and will attack using both hands for his attack option. Simply select both lines for Longsword and Shortsword.
If the creature has multiple attacks on the same line, you can individually select which attack bonuses you will be using by pressing on the grey circle with the attack bonus number to select that only.
After we pick our attacks, take a look at the Attack Adjustments. Use this if you have a special feat or trait that gives you a bonus for a particular fighting style not reflected on your d20Pro character sheet. You may also select if you have a Flank bonus (combat advantage) which will apply a +2 bonus to attack. If everything is good, press the "OK" button to select your target. If you have multiple attacks selected, multiple target submittion will ocurr.
There will be a time in your character's life where you will have multiple lines of attacks on multiple pages. To access, simply click the left or right arrow button, or use your mouse scroll-wheel to flip between the pages of attacks.
Here is an example of only picking certain attacks from a complete attack line. Simply just click on the grey circle with the attack bonus you wish to use for the attack line. In this case, there are three bad guys adjacent which this barbarian wants to use his cleave feat on.
And that's all there is needed to know about selecting attacks, selecting targets, and submitting the attack to the Judge/GM.