The RAS SDK is very powerful and can be used to do almost anything you can do in the Crystal Reports designer.
Sometimes the naming of methods can be confusing, but overall it is not overly difficult to find what you are looking for (assuming it exists).
I had a customer looking to modify their report parameters prompt text as well as the descriptions of default values and was running into issues.
I will post the code I used, but there are 2 things that you need to know.
1) The prompt text of a parameter is modified using the .Description off of the ParameterField object. This is confusing since you use a .Description method off of the ParameterFieldDiscreteValue to change the default value description. I wish it was .PromptText instead...oh well.
2) You can NOT modify dynamic parameter information..period. Even trying to change the prompting text will result in an error stating that dynamic parameters can not be modified.
Here is the code I used to update the parameters prompt text and the descriptions of a couple default values of the parameter.
VB.NET sample code
I hope this helps!
Good luck with your project!