Make sure that your Cache Method or ClassMethod has the [ ZenMethod ] keyword.

If you're within a %ZEN.Component.page this should work:
Method MyMethod() [ ZenMethod ]
{
    // Within ZEN Pages
    // Set with client code
   &js<
      zenPage.currentTableRow = "test"
   >
  // Set with server code
  S %page.currentTableRow = "test"
}

If you're within a %ZEN.Component.composite and the variable is also within the composite, this should work:
Method MyMethod() [ ZenMethod ]
{
  // Within Composite Components, here's one way that works as long as your <composite> has an id
  // Set with client code
  S ID = ..id
   &js<
      zen("#(ID)#").currentTableRow = "test"
   >
   // Set with server code
   S $this.currentTableRow = "test"
}

I left out the ClientMethod versions for brevity, but basically just use zenPage.currentTableRow within ZenPages, and this.currentTableRow within Composite Components.

Hope this helps!