SSRS: Get values from a particular row of DataSet?

In this example I have a tablix with Statecode and name as below enter image description here

Suppose you want to display the name of state of CA, write an expression as -

=Lookup(
   "CA" ,
   Fields!StateCode.Value,
   Fields!StateName.Value,
   "ReportData"
)

This will return 'California' in the text box

I hope you are using SSRS 2008R2:

R2 introduced the Lookup function that is perfect for this scenario.

=Lookup( Fields!ProductUID.Value ,Fields!ProductID.Value,Fields!Price.Value,"PriceDataSet")

The Lookup function above will evaluate the first parameter ("Fields!ProductUID.Value") in the current dataset, then look for a matching value in the field specified in the second parameter ("Fields!ProductID.Value") in the dataset specified in the fourth parameter. The value of the third parameter is then evaluated in that row of the dataset and returned.

A little convoluted, but very helpful.

In your case, you can use this in a textbox with a calculated a static number:

=Lookup(
    Month(DateAdd(DateInterval.Month, -1, GetDate())),
    Fields!MonthID.Value,
    Fields!Name.Value,
    "DataSet1")

This should calculate a number for last month, then look for a match in DataSet1.