In Adobe Photoshop CS5 you can not use Khmer OS Font. You need to use Kh Font, but for new version of Adobe CS6, How to use khmer unicode in adobe photoshop cs6.
In the case you can’t find the sign of Middle Eastern paragraph, please follows some step:
Step 1: Go to Edit => Preference => Type
Step 2: After that click on Middle Eastern
Step 3: Re-Open Adobe Photoshop CS6