There is a document that comes with XPP that covers all of the basics. Here
is a <a href="xpp_doc.pdf">copy</a>
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Some notes on the homework:
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In problem 2 take $V_{th}>V_r>V_{reset}$, $V_{th}-V_r>V_r-V_{reset}$. Resetting of voltage corresponds to firing. 
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a) For the f-I curve take, for example, $g=1$, $V_r=0$, $V_{th}=1$, $V_{reset}=0$.
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b) To answer the last question, think about the behavior of $f$ for large $I$. 
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c) You can use the following XPP code: <a href="IF.ode">IF.ode</a>. 
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3. The following code comes with XPP: <a href="ML.ode">ML.ode</a>. Feel free to use it. 
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4. Here you go: <a href="square.ode">Chay-Keizer</a>. THis is set up with z (a function of calcium cincentration as a parameter). To run the full system. Uncomment Ca equation, initial condition, and the expression for z, and comment p z=0