How to write a string to a file and store it locally in IS 6.1?

There is no built in write to file service in IS. You need to write a Java service which writes locally. There is a working example in the PSUtilities package.

Write a simple java service with string handling functionalities.

To emphasize what mwroblinski mentioned, there are services available for you to use. There is no need to write your own Java service for this.

As I mentioned PSUtilities I would like to add that I wouldn’t recommend to use the serive directly in PS Utilities.
PSUtilities is a collection of services written by PS people and given away to add functionality missing in the core packages. Those services are not quality assured an lot are quite old and do not resemble state of the art.

My advice:
Don’t deploy PSUtilities to a productive environment.
If you find a service in PSUtilities useful make a copy to a dedicated package holding those services you really use, check it’s funtionality and quality, if needed enhance it and use this package.
This way you can easily collect missing functionality but for the you are aware which services are used and have some kind of quality assurance.

Agreed 100%.