I have (supposedly) successfully installed xypic and am attempting to draw some diagrams. Everything is compiling just fine, but no matter what I do I cannot get the heads of the arrows to appear. All that shows up is the base of the arrow. For instance,

\begin{xy}
{\ar (0,0)*+{A}; (25,0)*+{A}};
\end{xy}


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Are you sure this is not an artifact of your viewer? What does the arrow look like if you open the PDF in Adobe Reader? – Alan Munn Apr 25 '11 at 23:27
Thank you for your responses. I tried both of these already and it's still not including the arrowheads. I was thinking I might have place some of the files in the wrong directory. For instance, I couldn't come up with a natural place to put the Metafont (.mf) files in my ~/Library/texmf directory. Any suggestions? – Brian Apr 26 '11 at 1:13
Since you're using a Mac, why did you have to install xypic in the first place? It should be part of your distribution. Maybe you should remove everything you installed in ~/Library/texmf and try it again. Your sample code works fine for me on a Mac, without having installed anything other than a full MacTeX. – Alan Munn Apr 26 '11 at 1:45
It worked, thank you! – Brian Apr 27 '11 at 3:59
@Jasper: if would be good if you would post that comment as an answer, additionally. – Stefan Kottwitz May 6 '11 at 21:22

Use \usepackage[all,dvips]{xy} or \usepackage[all,pdftex]{xy} in the preamble according to whether you compile the diagram via latex+dvips+ps2pdf or pdflatex.
I had this problem, and the dvips/pdftex fix didn't work for me. However, I eventually got it working by making sure I included \usepackage{amsfonts, amsmath, amssymb} in my preamble. Worth a try :)