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.

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 in most cases you use 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 may show up before nightfall.

She hadn't shown up before supper was ready, but they decided to wait a little longer for her in case she did show up.

As with other verbs, you can use the simple present to describe a regular occurrence:

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