Insert a template instrument through the menu Insert Instrument template or by pressing Alt+i twice. Set up an instrument, consisting only of an arm (keep the arm 'Progress_bar' that is already in the file), a source (Source_Maxwell_3) and two monitors (a PSD_monitor and an L_monitor). Insert the source at (0,0,0) relative to the origin and the monitors at (0,0,1) relative to the origin. As you input each component you should read the their documentation to find the needed input parameters. The component library can be accessed by clicking the Help (McDoc) button on McGui and choosing 'Component library index'. This will open up an webpage with a list of all the components.
For the source we will help you out. Try
COMPONENT source = Source_Maxwell_3(
size = 0.1, l_low = 0.1, l_high = 10, dist = 10, xw = 0.01,
yh = 0.01, T1 = 300, T2=300, T3=300, I1=1e14, I2=0, I3=0)
AT (0, 0, 0) RELATIVE Origin
Read the Source_Maxwell_3 documents using McDoc to understand the suggested parameters.