Is your new field simply not appearing in the list of fields from which to choose? What is the data type of the new field you added and calculated; text or numeric? Raster grids in ArcGIS need to use integer or floating-point values for the attribute in the raster attribute table. Thus, you cannot use a text field in a vector attribute table for the cell assignment in converting vector data to raster data. If your new field is TEXT, try changing it to numeric (e.g., DOUBLE, FLOAT, LONG) when adding it and prior to calculating it.