My suggestion is to use the calc library (see the pgfmanual 13.5 Coordinate Calculations - version October 25, 2010).
An example:
\documentclass{article} \usepackage{tikz} \usetikzlibrary{calc} \begin{document} \begin{tikzpicture} \node (a) {a}; \node (c) [right of=a] {c}; \node (b) at ($(a)!0.5!(c)$) {b}; \end{tikzpicture} \end{document} Notice that the syntax you used is wrong: each node should end with ; and the node c can not be positioned at its right.