Make sure the package is mounted. There are two tcl procs clickthrough_href and clickthrough_link and they have ad_proc documentation defined.
ad_proc clickthrough_href {href_url} {
Detects if a clickthrough instance is mounted beneath the current package
(ie, if the current package is interested in tracking clickthroughs).
Builds the href part of a clickthrough-enabled (or plain) HTML link based on the presence (or not) of such a clickthrough instance.
ad_proc clickthrough_link {href_url link_content {link_attributes ""}} {
Detects if a clickthrough instance is mounted beneath the current package
(ie, if the current package is interested in tracking clickthroughs).
Builds a full clickthrough-enabled (or plain) HTML link based on the presence (or not) of such a clickthrough instance.
You'll have to write some code to substitute links with these URLs in your weblog.