Install the software synthesizer you want to use. Make sure it is compatible with your operating system and that it includes the RTAS and/or TDM format, depending on which version of Pro Tools you are using. This process will vary depending on your platform (Mac or PC) and the installation style of the plug-in in question.
Open Pro Tools. It will find, register and validate the new software synthesizer plug-in automatically, and add it to your list of available plug-ins within Pro Tools.
Create a new Instrument track. When its channel strip appears, click on an empty insert slot under "INSERT A-E." A pop-up menu listing all of your available plug-ins of each type -- effects and instruments sorted under their respective RTAS or TDM categories -- will appear.
Go to the "Instruments" sub-menu. If your software synthesizer is in RTAS format, it will be under either "multichannel RTAS plug-ins" or "multi-mono RTAS plug-ins." TDM plug-ins will be similarly categorized in sub-folders. Sub-folder names will vary between systems because each user will have a different version of Pro Tools and/or different plug-ins installed. When you locate the name of your new software synthesizer, select it. It will be inserted on the new track and its interface will appear. You are now ready to use your new software synthesizer in any project.