The problem here is more logical than linguistic.  "Showing up" is a *state* and not an *ongoing action* -- you can either show up or not show up, but you can't "be showing up", so this is expressed using the past or the future tense:

> She **didn't** show up yesterday or today, and she probably **will not show up** tomorrow.

> She **hasn't shown up** yet, but I think she **will show up** before nightfall.

> She **hadn't shown up** before supper was ready, but they decided to wait a little longer for her.

As with other verbs, you can use the simple present to describe something that occurs regularly:

> She **normally shows up** by this time, but today she's late.