Go and get this .pdf from AutoDirect's website
https://cdn.automationdirect.com/static/manuals/surestepmanual/scl_manual.pdfit tells you all of the commands on the first few pages but the Absolute move command is "FP" on page 30 of the document or "FL" for a relative move pg 27
"FPXXXXXX" where the x is the number of motor steps you want to move.
It looks like this stepper controller works in raw counts so if you wanted to move by Engineering units like mm or in then you'd have to do some math.
First determine how many motor steps it takes to move 1 engineering unit you want, let's take Inches (assuming you're using a linear stage and not a rotary chuck).
Most motors have 200 count/rev or steps/rev without Micro stepping turned on. So lets say it takes 1000 steps to move the stage 1 linear inch then it's 1/1000 = 0.001" per motor step.
We don't know what interface you're trying to do but if you gave the operator a Text Box which is a standard control in Visual Studio and this was where they could type in the distance they wanted to go, you would simply convert it from a String to an integer and do math on it to get the number of steps to move.
Dim steps as Integer = Convert.ToInt16(Me.TextBox1.Text) / 0.001
You could do this on the TextChanged event of the textbox so it's always calculating the steps amount after they are done entering the value.
Public steps as Integer = 0
Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged
steps = Convert.ToInt16(Me.TextBox1.Text) / 0.001
End Sub
Then if you wanted to have a button transmit that command to the stepper driver, you could do add a standard button to your form, then double click it to edit the Click event for the button.
Add some code like this in conjunction with what Archie has already told you above.
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Me.SerialPort1.Open()
Me.SerialPort1.WriteLine(steps)
Me.SerialPort1.Close()
End Sub
This is all incredibly crude but it would work.