I updated MiKTeX and everything. I get this:
(C:\Users\Bora\Desktop\aaa\12.23.2011.aux)Trying to make PK font yrcmex10 at 600 DPI...
Running miktex-makemf.exe...
miktex-makemf: The yrcmex source file could not be found.
Running ttf2pk.exe...
miktex-makepk: PK font yrcmex10 could not be created.
)
(\end occurred inside a group at level 1)
### semi simple group (level 1) entered at line 67 (\begingroup)
### bottom level
!pdfTeX error: miktex-pdftex.exe (file yrcmex10): Font yrcmex10 at 600 not foun
d
==> Fatal error occurred, no output PDF file produced!
The MWE is:
\documentclass[12pt]{article}
\newcommand{\BEL}[1]{\text{\boldmath ${\mathsf{L}\!\negmedspace\negmedspace\mathsf{L}}_{#1}$}}
\usepackage{yhmath}
\usepackage{bm}
\begin{document}
\[ \big((\forall y \in x)A^{\BEL{\alpha}}(x|y) A^{\BEL{\alpha}}\big) \]
\end{document}
My directories are like this:

yhcmex10.mfand rename it asyrcmex10.mf(put it in the same directory...\fonts\source\public\yhmathor equivalent for.mffiles) and refresh the filename database. Try again and tell me. – Dec 26 '12 at 02:30