Use those functions:
int i2d_ECDSA_SIG(const ECDSA_SIG *sig, unsigned char **pp);
ECDSA_SIG* d2i_ECDSA_SIG(ECDSA_SIG **sig, const unsigned char **pp, long len);
As per OpenSSL docs:
" i2d_ECDSA_SIG() creates the DER encoding of the ECDSA signature sig and writes the encoded signature to *pp (note: if pp is NULL i2d_ECDSA_SIG returns the expected length in bytes of the DER encoded signature). i2d_ECDSA_SIG returns the length of the DER encoded signature (or 0 on error).
d2i_ECDSA_SIG() decodes a DER encoded ECDSA signature and returns the decoded signature in a newly allocated ECDSA_SIG structure. *sig points to the buffer containing the DER encoded signature of size len. "
If you want to printout the signature on the screen than you will have to Base64 it :)