Before you do this make sure there are no running instances of TextEdit on your system.
Using The Finder
Select the plist file in the Finder and doing File > Get Info. At the bottom of the file info window you'll see a section called Sharing & Permissions. You want to make sure you're in that list and that you have
Read & Write level access to the file.
Before you can add yourself to the access permissions on the file though, you'll need to unlock it. Click the little lock icon (1) in the lower right hand corner and enter your password. If you're not an administrator on the machine, you won't be able to proceed past this step.
Once you've unlocked the file you can change your permissions to
Read & Write. If you're not shown in the list of people who can access the file, click the
+ button below the list and find yourself in the
Users & Groups list that pops up.
The changes are applied to the file as soon as you make them in the File Info window.
You should now be able to open, edit and save those edits to the file.
Using The Command Line
Open a Terminal window.
Change to the place where the plist file is located:
Change the ownership and permissions on the file so you can read and write it:
sudo chown $USER aomDSP.plist
sudo chmod u+w aomDSP.plist
You probably don't need
sudo on the second call there, but it can't hurt. You'll need administrator access to the machine to do this.