Easiest by car to follow M0, M1, M6 or M7 highway from Budapest and leave at the exit of Érd. Routes 6 and 70 also cross through the city.
Trains leave most often from Budapest Déli station (this train will also stop in Budapest Kelenföld). Note that Érd has three stations: Érd felső (upper Érd), Érd, and Érd alsó (lower Érd). Not all the trains stop in all three so check before boarding. Journey time will be roughly 30 to 40 minutes. Check  for the exact schedule.
There are a few options by bus. The easiest to take the frequent-running bus from Etele tér which will take you directly to Érd in about 25-45 minutes. There are less frequent buses that leave from Csepel, or you can also take the #33 from Móricz Zsigmond körtér in Buda to Nagytetény, and from there catch a 10 minute bus to Érd.