where is Defn.h and implementation

I am in the process of writing my first R extesion package.

Have you read our posting guide?  Please ask C programming questions on 
the apppropriate list -- R-devel.

Have you read `Writing R Extensions'?  It does cover this topic with 
copious warnings against what you are asking to do.

One of my function uses StrToInternal function(I copy it from other
extension package).

Which package is that?  Please avoid copying from packages which use
internal headers (and they probably do not work).

I noticed the StrToInternal was defined in "Defn.h".
I have two questions:
1)should "Defn.h"  be under $R_HOME/include. I could not find it in any
file under $R_HOME/include.

No.  It is an internal header.  You could have read the Makefiles to see 
that it is not an exported header.

2)Where is the implementation for the functions defined in the "Defn.h"?

Almost always in src/main/*.c.

Where dynamic library should I link to for the StrToInternal implementation?

None, you should not be calling functions not documented in `Writing R 
Extensions', and you certainly should not be asking for technical support 
to do so.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595