I'll go through this from start to finish. You may already have most of this done correctly.
When you set up a Pay Item in the Payroll module, Payroll / Setup / Maintain Item Descriptions / ADD / Employer Liability, there are row headings that say: "liability account" and "expense account".
These row headings are generally accurate. Therefore, you should enter a liability account and expense account. When the payroll check is cut, both the expense account and liability account are increased.
CR 01-2132-000 liability $50
DB 01-5132-000 expense $50
To pay off the liability, you cut a check from Accounts Payable to your bank, or whoever handles the employer liability portion (in Accounts Payable)
CR 01-1110-000 checking $50
DB 01-2132-000 liability $50
The liability account will be "zeroed-out" for this one payroll item, and the expense was already recorded when the payroll check was first cut.