That library uses constructs that aren't valid according to the Modelica specification. However, you can use it in SystemModeler if you use the external objects and their functions directly, like this adapted XLSTest for example:
model XLSTest "Excel XLS file read test"
extends Modelica.Icons.Example;
parameter Types.ExternXLSFile xls = Types.ExternXLSFile(Modelica.Utilities.Files.loadResource("modelica://ExternData/Resources/Examples/test.xls"), "UTF-8", true);
Modelica.Blocks.Math.Gain gain1(k = Functions.XLS.getReal("B2", "set1", xls)) annotation(Placement(transformation(extent = {{-15, 60}, {5, 80}})));
Modelica.Blocks.Math.Gain gain2(k = Functions.XLS.getReal("B2", "set2", xls)) annotation(Placement(transformation(extent = {{-15, 30}, {5, 50}})));
Modelica.Blocks.Sources.Clock clock annotation(Placement(transformation(extent = {{-50, 60}, {-30, 80}})));
equation
connect(clock.y, gain1.u) annotation(Line(points = {{-29, 70}, {-17, 70}}, color = {0, 0, 127}));
connect(clock.y, gain2.u) annotation(Line(points = {{-29, 70}, {-22, 70}, {-22, 40}, {-17, 40}}, color = {0, 0, 127}));
annotation(experiment(StopTime = 1), Documentation(info = "<html><p>This example model reads the gain parameters from different cells and sheets of the Excel XLS file <a href=\"modelica://ExternData/Resources/Examples/test.xls\">test.xls</a>. For gain1 the gain parameter is read as Real value using the function <a href=\"modelica://ExternData.XLSFile.getReal\">ExternData.XLSFile.getReal</a>. For gain2 the String value is retrieved by function <a href=\"modelica://ExternData.XLSFile.getString\">ExternData.XLSFile.getString</a> and converted to a Real value (using the utility function <a href=\"modelica://Modelica.Utilities.Strings.scanReal\">Modelica.Utilities.Strings.scanReal</a>). For timeTable the table parameter is read as Real array of dimension 3x2 by function <a href=\"modelica://ExternData.XLSFile.getRealArray2D\">ExternData.XLSFile.getRealArray2D</a>. The read parameters are assigned by parameter bindings to the appropriate model parameters.</p></html>"));
end XLSTest;
Otto